revert: be more helpful on uncommitted merges

 BEFORE:
   $ hg revert
   abort: no files or directories specified
   (use --all to discard all changes)

 AFTER:
   $ hg revert
   abort: no files or directories specified
   (uncommitted merge, use --all to discard all changes, or 'hg update -C .' to abort the merge)

mercurial/commands.py

@@ -4181,8 +4181,12 b' def revert(ui, repo, *pats, **opts):'
     parent, p2 = repo.dirstate.parents()
 
     if not pats and not opts.get('all'):
-        raise util.Abort(_('no files or directories specified'),
-                         hint=_('use --all to discard all changes'))
+        msg = _("no files or directories specified")
+        hint = _("use --all to discard all changes")
+        if p2 != nullid:
+            hint = _("uncommitted merge, use --all to discard all changes,"
+                     " or 'hg update -C .' to abort the merge")
+        raise util.Abort(msg, hint=hint)
 
     ctx = scmutil.revsingle(repo, opts.get('rev'))
     node = ctx.node()

tests/test-confused-revert.t

@@ -60,7 +60,7 b' Revert should fail:'
 
   $ hg revert
   abort: no files or directories specified
-  (use --all to discard all changes)
+  (uncommitted merge, use --all to discard all changes, or 'hg update -C .' to abort the merge)
   [255]
 
 Revert should be ok now: