[BUG] rev-list doesn't validate arguments to -n option

It tolerates non-numeric arguments and garbage after a number:

For example:

$ # -n 1 means same as -n 0:
$ git rev-list -n q newest_commit
$ git rev-list -n 0 newest_commit
$ # Garbage after number is tolerated:
$ git rev-list -n 1q newest_commit
3be33f83695088d968cf084a1a08bdcde25a8d7a
$ git rev-list -n 2q newest_commit
3be33f83695088d968cf084a1a08bdcde25a8d7a
286e62e1b68e39334978e6222cbff187ecc17bcf