diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -2805,6 +2805,12 @@
         fparent1 = manifest1.get(fname, nullid)
         fparent2 = manifest2.get(fname, nullid)
         if isinstance(fctx, context.filectx):
+            # This block fast path most comparison usually done, It is making
+            # the assumption that a bare filectx is used, and that no merge
+            # happened on this file.
+            #
+            # It assume they are no need to a create a new file revision in
+            # this case.
             node = fctx.filenode()
             if node in [fparent1, fparent2]:
                 self.ui.debug(b'reusing %s filelog entry\n' % fname)