Instead of hard-coding the offset strlen("refs/heads/") to skip
the prefix "refs/heads/" use the skip_prefix() function which
is more communicative and verifies that the string actually
starts with that prefix.
Signed-off-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>