refs: implement simple transactions for the packed-refs file

refs: implement simple transactions for the packed-refs file

Handle simple transactions for the packed-refs file at the
packed_ref_cache level via new functions lock_packed_refs(),
commit_packed_refs(), and rollback_packed_refs().

Only allow the packed ref cache to be modified (via add_packed_ref())
while the packed refs file is locked.

Change clone to add the new references within a transaction.

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