Index: hgext/rebase.py
===================================================================
--- hgext/rebase.py
+++ hgext/rebase.py
@@ -887,6 +887,28 @@
 
         rbsrt._finishrebase()
 
+def _shoulddisableimm(ui, repo, rebaseset):
+    """returns if we should disable in-memory merge based on the rebaseset"""
+    # If rebasing the working copy parent, force in-memory merge to be off.
+    #
+    # This is because the extra work of checking out the newly rebased commit
+    # outweights the benefits of rebasing in-memory, and executing an extra
+    # update command adds a bit of overhead, so better to just do it on disk. In
+    # all other cases leave it on.
+    #
+    # Note that there are cases where this isn't true -- e.g., rebasing large
+    # stacks that include the WCP. However, I'm not yet sure where the cutoff
+    # is.
+    rebasingwcp = repo['.'].rev() in rebaseset
+    ui.log("rebase", "", rebase_rebasing_wcp=rebasingwcp)
+    if rebasingwcp:
+        whynotimm = "wcp in rebaseset"
+        ui.log("rebase", "disabling IMM because: %s" % whynotimm,
+            why_not_imm=whynotimm)
+        return True
+
+    return False # no change
+
 def _definedestmap(ui, repo, rbsrt, destf=None, srcf=None, basef=None,
                    revf=None, destspace=None):
     """use revisions argument to define destmap {srcrev: destrev}"""
@@ -977,23 +999,10 @@
                 ui.status(_('nothing to rebase from %s to %s\n') %
                           ('+'.join(str(repo[r]) for r in base), dest))
             return None
-    # If rebasing the working copy parent, force in-memory merge to be off.
-    #
-    # This is because the extra work of checking out the newly rebased commit
-    # outweights the benefits of rebasing in-memory, and executing an extra
-    # update command adds a bit of overhead, so better to just do it on disk. In
-    # all other cases leave it on.
-    #
-    # Note that there are cases where this isn't true -- e.g., rebasing large
-    # stacks that include the WCP. However, I'm not yet sure where the cutoff
-    # is.
-    rebasingwcp = repo['.'].rev() in rebaseset
-    ui.log("rebase", "", rebase_rebasing_wcp=rebasingwcp)
-    if rbsrt.inmemory and rebasingwcp:
+
+    # Possibly disable in-memory merge based on the rebaseset.
+    if rbsrt.inmemory and _shoulddisableimm(ui, repo, rebaseset):
         rbsrt.inmemory = False
-        whynotimm = "wcp in rebaseset"
-        ui.log("rebase", "disabling IMM because: %s" % whynotimm,
-            why_not_imm=whynotimm)
 
         # Check these since we did not before.
         cmdutil.checkunfinished(repo)