cherry-pick: do not expect file arguments

cherry-pick: do not expect file arguments

If a commit-ish passed to cherry-pick or revert happens to have a file
of the same name, git complains that the argument is ambiguous and
advises to use '--'. To make things worse, the '--' argument is removed
by parse_options, und so passing '--' has no effect.

Instead, always interpret cherry-pick/revert arguments as revisions.

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>