diff: use cmdutil.check_at_most_one_arg() for checking --rev/--change

The same check was done in extdiff as well, so I fixed that too.

There are apparently no tests for this.

Differential Revision: https://phab.mercurial-scm.org/D8510

hgext/extdiff.py

@@ -360,14 +360,12 b' def dodiff(ui, repo, cmdline, pats, opts'
     - just invoke the diff for a single file in the working dir
     '''
 
+    cmdutil.check_at_most_one_arg(opts, b'rev', b'change')
     revs = opts.get(b'rev')
     change = opts.get(b'change')
     do3way = b'$parent2' in cmdline
 
-    if revs and change:
-        msg = _(b'cannot specify --rev and --change at the same time')
-        raise error.Abort(msg)
-    elif change:
+    if change:
         ctx2 = scmutil.revsingle(repo, change, None)
         ctx1a, ctx1b = ctx2.p1(), ctx2.p2()
     else:

mercurial/commands.py

@@ -2475,16 +2475,14 b' def diff(ui, repo, *pats, **opts):'
     Returns 0 on success.
     """
 
+    cmdutil.check_at_most_one_arg(opts, 'rev', 'change')
     opts = pycompat.byteskwargs(opts)
     revs = opts.get(b'rev')
     change = opts.get(b'change')
     stat = opts.get(b'stat')
     reverse = opts.get(b'reverse')
 
-    if revs and change:
-        msg = _(b'cannot specify --rev and --change at the same time')
-        raise error.Abort(msg)
-    elif change:
+    if change:
         repo = scmutil.unhidehashlikerevs(repo, [change], b'nowarn')
         ctx2 = scmutil.revsingle(repo, change, None)
         ctx1 = ctx2.p1()