rebase: use cmdutil.check_at_most_one_arg() for action

Here we also needed to know what the action was (if any), so I've
updated the helper to return any specified option.

Differential Revision: https://phab.mercurial-scm.org/D7640

hgext/rebase.py

@@ -1021,12 +1021,7 b' def rebase(ui, repo, **opts):'
     inmemory = ui.configbool(b'rebase', b'experimental.inmemory')
     dryrun = opts.get(b'dry_run')
     confirm = opts.get(b'confirm')
-    selactions = [k for k in [b'abort', b'stop', b'continue'] if opts.get(k)]
-    if len(selactions) > 1:
-        raise error.Abort(
-            _(b'cannot use --%s with --%s') % tuple(selactions[:2])
-        )
-    action = selactions[0] if selactions else None
+    action = cmdutil.check_at_most_one_arg(opts, b'abort', b'stop', b'continue')
     if dryrun and action:
         raise error.Abort(_(b'cannot specify both --dry-run and --%s') % action)
     if confirm and action:

mercurial/cmdutil.py

@@ -261,7 +261,10 b' debugrevlogopts = ['
 
 
 def check_at_most_one_arg(opts, *args):
-    """abort if more than one of the arguments are in opts"""
+    """abort if more than one of the arguments are in opts
+
+    Returns the unique argument or None if none of them were specified.
+    """
     previous = None
     for x in args:
         if opts.get(x):
@@ -270,6 +273,7 b' def check_at_most_one_arg(opts, *args):'
                     _(b'cannot specify both --%s and --%s') % (previous, x)
                 )
             previous = x
+    return previous
 
 
 def check_incompatible_arguments(opts, first, *others):

tests/test-rebase-obsolete.t

@@ -2062,7 +2062,7 b' Test --stop raise errors with conflictin'
   (use 'hg rebase --continue' or 'hg rebase --abort')
   [255]
   $ hg rebase --stop --continue
-  abort: cannot use --stop with --continue
+  abort: cannot specify both --stop and --continue
   [255]
 
 Test --stop moves bookmarks of original revisions to new rebased nodes:

tests/test-rebase-parameters.t

@@ -61,7 +61,7 b' These fail:'
   [1]
 
   $ hg rebase --continue --abort
-  abort: cannot use --abort with --continue
+  abort: cannot specify both --abort and --continue
   [255]
 
   $ hg rebase --continue --collapse