diff --git a/mercurial/filemerge.py b/mercurial/filemerge.py
--- a/mercurial/filemerge.py
+++ b/mercurial/filemerge.py
@@ -421,8 +421,6 @@
             )
 
     if premerge:
-        if len(labels) < 3:
-            labels.append(b'base')
         mode = b'merge'
         if premerge == b'keep-mergediff':
             mode = b'mergediff'
@@ -473,14 +471,11 @@
     ui = repo.ui
 
     local = simplemerge.MergeInput(fcd)
-    if len(labels) > 0:
-        local.label = labels[0]
+    local.label = labels[0]
     other = simplemerge.MergeInput(fco)
-    if len(labels) > 1:
-        other.label = labels[1]
+    other.label = labels[1]
     base = simplemerge.MergeInput(fca)
-    if len(labels) > 2:
-        base.label = labels[2]
+    base.label = labels[2]
     r = simplemerge.simplemerge(ui, local, base, other, mode=mode)
     return True, r, False
 
@@ -535,8 +530,6 @@
     files. It will fail if there are any conflicts and leave markers in
     the partially merged file. Marker will have three sections, one from each
     side of the merge and one for the base content."""
-    if len(labels) < 3:
-        labels.append(b'base')
     return _merge(repo, fcd, fco, fca, labels, b'merge3')
 
 
@@ -575,8 +568,6 @@
     the partially merged file. The marker will have two sections, one with the
     content from one side of the merge, and one with a diff from the base
     content to the content on the other side. (experimental)"""
-    if len(labels) < 3:
-        labels.append(b'base')
     return _merge(repo, fcd, fco, fca, labels, b'mergediff')
 
 
@@ -736,10 +727,7 @@
         basepath, otherpath, localoutputpath = temppaths
         outpath = b""
         mylabel, otherlabel = labels[:2]
-        if len(labels) >= 3:
-            baselabel = labels[2]
-        else:
-            baselabel = b'base'
+        baselabel = labels[2]
         env = {
             b'HG_FILE': fcd.path(),
             b'HG_MY_NODE': short(mynode),
@@ -861,9 +849,8 @@
     newlabels = [
         _formatlabel(cd, tmpl, labels[0], pad),
         _formatlabel(co, tmpl, labels[1], pad),
+        _formatlabel(ca, tmpl, labels[2], pad),
     ]
-    if len(labels) > 2:
-        newlabels.append(_formatlabel(ca, tmpl, labels[2], pad))
     return newlabels
 
 
@@ -1063,6 +1050,8 @@
 
     if not labels:
         labels = [b'local', b'other']
+    if len(labels) < 3:
+        labels.append(b'base')
     if mergetype == nomerge:
         return func(repo, mynode, fcd, fco, fca, toolconf, labels)
 
diff --git a/tests/test-annotate.t b/tests/test-annotate.t
--- a/tests/test-annotate.t
+++ b/tests/test-annotate.t
@@ -225,7 +225,7 @@
   b4
   c
   b5
-  ||||||| base
+  ||||||| base:         3086dbafde1c - test: b
   =======
   b4
   b5
@@ -750,7 +750,7 @@
   <<<<<<< working copy: 863de62655ef - test: baz:3+->3-
   3- baz:3
   4 baz:4
-  ||||||| base
+  ||||||| base:         56fc739c091f - test: baz:3->3+
   3+ baz:3
   4 baz:4
   =======
@@ -797,7 +797,7 @@
   <<<<<<< working copy: cb8df70ae185 - test: qux:4->4+
   3+ baz:3
   4+ baz:4
-  ||||||| base
+  ||||||| base:         56fc739c091f - test: baz:3->3+
   3+ baz:3
   4 baz:4
   =======
@@ -1176,7 +1176,7 @@
   1
   2
   3
-  ||||||| base
+  ||||||| base:         1ed24be7e7a0 - test: 2
   1
   2
   =======
diff --git a/tests/test-diff-change.t b/tests/test-diff-change.t
--- a/tests/test-diff-change.t
+++ b/tests/test-diff-change.t
@@ -284,7 +284,7 @@
    7
   -<<<<<<< local: fd1f17c90d7c - test: new file
    z
-  -||||||| base
+  -||||||| base:  ae119d680c82 - test: lots of text
   -8
   -=======
   -y
diff --git a/tests/test-fastannotate-hg.t b/tests/test-fastannotate-hg.t
--- a/tests/test-fastannotate-hg.t
+++ b/tests/test-fastannotate-hg.t
@@ -169,7 +169,7 @@
   b4
   c
   b5
-  ||||||| base
+  ||||||| base:         3086dbafde1c - test: b
   =======
   b4
   b5
@@ -793,7 +793,7 @@
   1
   2
   3
-  ||||||| base
+  ||||||| base:         1ed24be7e7a0 - test: 2
   1
   2
   =======
diff --git a/tests/test-merge-changedelete.t b/tests/test-merge-changedelete.t
--- a/tests/test-merge-changedelete.t
+++ b/tests/test-merge-changedelete.t
@@ -682,7 +682,7 @@
   3
   <<<<<<< working copy: 13910f48cf7b - test: changed file1, removed file2, chan...
   changed2
-  ||||||| base
+  ||||||| base:         ab57bf49aa27 - test: added files
   =======
   changed1
   >>>>>>> merge rev:    10f9a0a634e8 - test: removed file1, changed file2, chan...
diff --git a/tests/test-merge-force.t b/tests/test-merge-force.t
--- a/tests/test-merge-force.t
+++ b/tests/test-merge-force.t
@@ -400,7 +400,7 @@
   M content1_content2_content1_content4-tracked
   <<<<<<< working copy: 0447570f1af6 - test: local
   content4
-  ||||||| base
+  ||||||| base:         8ef80617fa20 - test: base
   content1
   =======
   content2
@@ -430,7 +430,7 @@
   M content1_content2_content2_content4-tracked
   <<<<<<< working copy: 0447570f1af6 - test: local
   content4
-  ||||||| base
+  ||||||| base:         8ef80617fa20 - test: base
   content1
   =======
   content2
@@ -460,7 +460,7 @@
   M content1_content2_content3_content3-tracked
   <<<<<<< working copy: 0447570f1af6 - test: local
   content3
-  ||||||| base
+  ||||||| base:         8ef80617fa20 - test: base
   content1
   =======
   content2
@@ -472,7 +472,7 @@
   M content1_content2_content3_content4-tracked
   <<<<<<< working copy: 0447570f1af6 - test: local
   content4
-  ||||||| base
+  ||||||| base:         8ef80617fa20 - test: base
   content1
   =======
   content2
@@ -502,7 +502,7 @@
   M content1_content2_missing_content4-tracked
   <<<<<<< working copy: 0447570f1af6 - test: local
   content4
-  ||||||| base
+  ||||||| base:         8ef80617fa20 - test: base
   content1
   =======
   content2
@@ -586,7 +586,7 @@
   M missing_content2_content2_content4-tracked
   <<<<<<< working copy: 0447570f1af6 - test: local
   content4
-  ||||||| base
+  ||||||| base:         000000000000 - : 
   =======
   content2
   >>>>>>> merge rev:    85100b8c675b - test: remote
@@ -609,7 +609,7 @@
   M missing_content2_content3_content3-tracked
   <<<<<<< working copy: 0447570f1af6 - test: local
   content3
-  ||||||| base
+  ||||||| base:         000000000000 - : 
   =======
   content2
   >>>>>>> merge rev:    85100b8c675b - test: remote
@@ -620,7 +620,7 @@
   M missing_content2_content3_content4-tracked
   <<<<<<< working copy: 0447570f1af6 - test: local
   content4
-  ||||||| base
+  ||||||| base:         000000000000 - : 
   =======
   content2
   >>>>>>> merge rev:    85100b8c675b - test: remote
@@ -643,7 +643,7 @@
   M missing_content2_missing_content4-tracked
   <<<<<<< working copy: 0447570f1af6 - test: local
   content4
-  ||||||| base
+  ||||||| base:         000000000000 - : 
   =======
   content2
   >>>>>>> merge rev:    85100b8c675b - test: remote
@@ -651,7 +651,7 @@
   M missing_content2_missing_content4-untracked
   <<<<<<< working copy: 0447570f1af6 - test: local
   content4
-  ||||||| base
+  ||||||| base:         000000000000 - : 
   =======
   content2
   >>>>>>> merge rev:    85100b8c675b - test: remote
diff --git a/tests/test-merge-tools.t b/tests/test-merge-tools.t
--- a/tests/test-merge-tools.t
+++ b/tests/test-merge-tools.t
@@ -1226,7 +1226,7 @@
   <<<<<<< working copy: ef83787e2614 - test: revision 1
   revision 1
   space
-  ||||||| base
+  ||||||| base:         ffd2bda21d6e - test: revision 0
   revision 0
   space
   =======
@@ -1242,7 +1242,7 @@
   <<<<<<< working copy: ef83787e2614 - test: revision 1
   revision 1
   space
-  ||||||| base
+  ||||||| base:         ffd2bda21d6e - test: revision 0
   revision 0
   space
   =======
@@ -1264,7 +1264,7 @@
   $ hg merge -r 4 --config merge-tools.true.premerge=keep-mergediff
   merging f
   <<<<<<<
-  ------- base
+  ------- base:         ffd2bda21d6e - test: revision 0
   +++++++ working copy: ef83787e2614 - test: revision 1
   -revision 0
   +revision 1
@@ -1280,7 +1280,7 @@
   $ aftermerge
   # cat f
   <<<<<<<
-  ------- base
+  ------- base:         ffd2bda21d6e - test: revision 0
   +++++++ working copy: ef83787e2614 - test: revision 1
   -revision 0
   +revision 1
diff --git a/tests/test-rebase-collapse.t b/tests/test-rebase-collapse.t
--- a/tests/test-rebase-collapse.t
+++ b/tests/test-rebase-collapse.t
@@ -719,7 +719,7 @@
   $ cat A
   <<<<<<< dest:   82b8abf9c185 D - test: D
   D
-  ||||||| base
+  ||||||| base:   4a2df7238c3b A - test: A
   A
   =======
   B
@@ -747,7 +747,7 @@
   $ cat A
   <<<<<<< dest:   82b8abf9c185 D - test: D
   BC
-  ||||||| base
+  ||||||| base:   f899f3910ce7 B - test: B
   B
   =======
   C
diff --git a/tests/test-rebase-conflicts.t b/tests/test-rebase-conflicts.t
--- a/tests/test-rebase-conflicts.t
+++ b/tests/test-rebase-conflicts.t
@@ -374,7 +374,7 @@
    a
   +<<<<<<< dest:   328e4ab1f7cc ab - test: ab
    b
-  +||||||| base
+  +||||||| base:   cb9a9f314b8b - test: a
   +=======
   +b
   +c
diff --git a/tests/test-update-branches.t b/tests/test-update-branches.t
--- a/tests/test-update-branches.t
+++ b/tests/test-update-branches.t
@@ -295,7 +295,7 @@
   <<<<<<< working copy: 6efa171f091b - test: 3
   three
   dirty
-  ||||||| base
+  ||||||| base:         6efa171f091b - test: 3
   three
   =======
   four