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 <redacted>
Signed-off-by: Junio C Hamano <redacted>