refs: do not execute reference-transaction hook on packing refs

refs: do not execute reference-transaction hook on packing refs

The reference-transaction hook is supposed to track logical changes to
references, but it currently also gets executed when packing refs in a
repository. This is unexpected and ultimately not all that useful:
packing refs is not supposed to result in any user-visible change to the
refs' state, and it ultimately is an implementation detail of how refs
stores work.

Fix this excessive execution of the hook when packing refs.

Reported-by: Waleed Khan <redacted>
Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>