refs.c: commit_packed_refs to return a meaningful errno on failure

refs.c: commit_packed_refs to return a meaningful errno on failure

Making errno when returning from commit_packed_refs() meaningful,
which should fix

 * a bug in "git clone" where it prints strerror(errno) based on
   errno, despite errno possibly being zero and potentially having
   been clobbered by that point
 * the same kind of bug in "git pack-refs"

and prepares for repack_without_refs() to get a meaningful
error message when commit_packed_refs() fails without falling into
the same bug.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Acked-by: Michael Haggerty <mhagger@alum.mit.edu>