Merge branch 'mh/ref-lock-entry'

The code to acquire a lock on a reference (e.g. while accepting a
push from a client) used to immediately fail when the reference is
already locked---now it waits for a very short while and retries,
which can make it succeed if the lock holder was holding it during
a read-only operation.

* mh/ref-lock-entry:
refs: retry acquiring reference locks for 100ms

diff --cc Documentation/config.txt
Simple merge

diff --cc refs.c
Simple merge

diff --cc refs/files-backend.c
Simple merge

diff --cc refs/refs-internal.h
Simple merge