The cherry-pick and revert commands use OPT_INTEGER() to
parse --mainline. The stock parser is smart enough to reject
non-numeric nonsense, but it doesn't know that parent
counting starts at 1.
Worse, the value "0" is indistinguishable from the unset
case, so a user who assumes the counting is 0-based will get
a confusing message:
$ git cherry-pick -m 0 $merge
error: commit ... is a merge but no -m option was given.
Let's use a custom callback that enforces our range.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>