backout: disable --merge with --no-commit (issue4874)

Because "backout --merge" have to make a commit before merging, it doesn't
work with --no-commit. We could change "backout --merge" to make a merge
commit automatically, and --no-commit to bypass a merge commit, but that
change would be undesirable because:

 a) it's hard to fix bad merges in general
 b) two commits would be created with the same --message

So, this patch simply disables "--merge --no-commit".

mercurial/commands.py

@@ -598,6 +598,8 b' def backout(ui, repo, node=None, rev=Non'
 def _dobackout(ui, repo, node=None, rev=None, **opts):
     if opts.get('commit') and opts.get('no_commit'):
         raise error.Abort(_("cannot use --commit with --no-commit"))
+    if opts.get('merge') and opts.get('no_commit'):
+        raise error.Abort(_("cannot use --merge with --no-commit"))
 
     if rev and node:
         raise error.Abort(_("please specify just one revision"))

tests/test-backout.t

@@ -753,3 +753,10 b' Test usage of `hg resolve` in case of co'
   $ hg backout --no-commit .
   removing 3
   changeset cccc23d9d68f backed out, don't forget to commit.
+  $ hg revert -aq
+
+--no-commit can't be used with --merge
+
+  $ hg backout --merge --no-commit 2
+  abort: cannot use --merge with --no-commit
+  [255]