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