merge: make error message consistent with other commands

If a merge is attempted when another merge is already ongoing, we give
the message "outstanding uncommitted merges". Many other commands
(such as backout, rebase, histedit) give the same message in singular
form. Since the singular form also seems to make more sense, let's use
that for 'hg merge' as well.

mercurial/merge.py

@@ -1058,7 +1058,7 b' def update(repo, node, branchmerge, forc'
 
         ### check phase
         if not overwrite and len(pl) > 1:
-            raise util.Abort(_("outstanding uncommitted merges"))
+            raise util.Abort(_("outstanding uncommitted merge"))
         if branchmerge:
             if pas == [p2]:
                 raise util.Abort(_("merging with a working directory ancestor"

tests/test-largefiles.t

@@ -1705,7 +1705,7 b' Test status after merging with a branch '
 - make sure update of merge with removed largefiles fails as expected
   $ hg rm sub2/large6
   $ hg up -r.
-  abort: outstanding uncommitted merges
+  abort: outstanding uncommitted merge
   [255]
 
 - revert should be able to revert files introduced in a pending merge