revert: drop requirement to use -r to revert with two parents

This reduces documentation confusion between the need to:

a) hg revert -a -r .  (drop all changes from a merge)
b) hg up -C .         (drop the second parent entirely)

Currently revert is one of two commands (the other being tag) that
still complains about uncommitted merges, dating from its former use
of a generic defaultrev function that aborted.

mercurial/commands.py

@@ -4093,11 +4093,9 b' def revert(ui, repo, *pats, **opts):'
        To cancel a merge (and lose your changes), use :hg:`update --clean .`.
 
     With no revision specified, revert the named files or directories
-    to the contents they had in the parent of the working directory.
+    to the contents they had in the first parent of the working directory.
     This restores the contents of the affected files to an unmodified
-    state and unschedules adds, removes, copies, and renames. If the
-    working directory has two parents, you must explicitly specify a
-    revision.
+    state and unschedules adds, removes, copies, and renames.
 
     Using the -r/--rev option, revert the given files or directories
     to their contents as of a specific revision. This can be helpful
@@ -4129,13 +4127,10 b' def revert(ui, repo, *pats, **opts):'
         opts["rev"] = cmdutil.finddate(ui, repo, opts["date"])
 
     parent, p2 = repo.dirstate.parents()
-    if not opts.get('rev') and p2 != nullid:
-        raise util.Abort(_('uncommitted merge - '
-                           'use "hg update", see "hg help revert"'))
 
     if not pats and not opts.get('all'):
-        raise util.Abort(_('no files or directories specified; '
-                           'use --all to revert the whole repo'))
+        raise util.Abort(_('no files or directories specified'),
+                         hint=_('use --all to revert all files'))
 
     ctx = scmutil.revsingle(repo, opts.get('rev'))
     node = ctx.node()

tests/test-confused-revert.t

@@ -59,7 +59,8 b' Should show a removed and b added:'
 Revert should fail:
 
   $ hg revert
-  abort: uncommitted merge - use "hg update", see "hg help revert"
+  abort: no files or directories specified
+  (use --all to revert all files)
   [255]
 
 Revert should be ok now:

tests/test-revert.t

@@ -185,7 +185,8 b' Issue241: update and revert produces inc'
 should fail - no arguments
 
   $ hg revert -rtip
-  abort: no files or directories specified; use --all to revert the whole repo
+  abort: no files or directories specified
+  (use --all to revert all files)
   [255]
 
 should succeed