merge: improve merge with ancestor message

mercurial/merge.py

@@ -467,7 +467,8 b' def update(repo, node, branchmerge, forc'
             raise util.Abort(_("outstanding uncommitted merges"))
         if branchmerge:
             if pa == p2:
-                raise util.Abort(_("can't merge with ancestor"))
+                raise util.Abort(_("merging with a working directory ancestor"
+                                   " has no effect"))
             elif pa == p1:
                 if p1.branch() != p2.branch():
                     fastforward = True

tests/test-issue619.out

@@ -6,5 +6,5 b' fast-forward'
 1 files updated, 0 files merged, 0 files removed, 0 files unresolved
 (branch merge, don't forget to commit)
 bogus fast-forward should fail
-abort: can't merge with ancestor
+abort: merging with a working directory ancestor has no effect
 done