Merge branch 'mh/lockfile-retry'

Instead of dying immediately upon failing to obtain a lock, retry
after a short while with backoff.

* mh/lockfile-retry:
  lock_packed_refs(): allow retries when acquiring the packed-refs lock
  lockfile: allow file locking to be retried with a timeout

diff --cc Documentation/config.txt
Simple merge

diff --cc refs.c
Simple merge