commit.c: use skip_prefix() instead of starts_with()
commit.c: use skip_prefix() instead of starts_with()

In record_author_date() & parse_gpg_output(), the callers of
starts_with() not just want to know if the string starts with the
prefix, but also can benefit from knowing the string that follows
the prefix.

By using skip_prefix(), we can do both at the same time.

Helped-by: Max Horn <0706025b2bbcec1ed8d64822f4eccd96314938d0@quendi.de>
Helped-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>
Helped-by: Michael Haggerty <c24c152f145c57791e809b126db9e61bd1c1782d@alum.mit.edu>
Signed-off-by: Tanay Abhra <43a0dd602247961f0b6e66a197d8f2daec3be19d@gmail.com>
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