cherry-pick: detect bogus arguments to --mainline

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>

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