fsck.c:fsck_commit(): use skip_prefix() to verify and skip constant

fsck.c:fsck_commit(): use skip_prefix() to verify and skip constant

fsck_commit() uses memcmp() to check if the buffer starts with a
certain prefix, and skips the prefix if it does.

This is exactly what skip_prefix() was designed for.

Signed-off-by: Yuxuan Shui <redacted>
Helped-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>