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 <redacted>
Helped-by: Junio C Hamano <redacted>
Helped-by: Michael Haggerty <redacted>
Signed-off-by: Tanay Abhra <redacted>
Signed-off-by: Junio C Hamano <redacted>