git: avoid magic number with skip_prefix
git: avoid magic number with skip_prefix

After handling options, any leftover arguments should be
commands. However, we pass through "--help" and "--version",
so that we convert them into "git help" and "git version"
respectively.

This is a straightforward use of skip_prefix to avoid a
magic number, but while we are there, it is worth adding a
comment to explain this otherwise confusing behavior.

Signed-off-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git