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

Documentation/config.txt

@@ -776,6 +776,12 @@ core.commentChar::
 If set to "auto", `git-commit` would select a character that is not
 the beginning character of any line in existing commit messages.
 
+core.filesRefLockTimeout::
+	The length of time, in milliseconds, to retry when trying to
+	lock an individual reference. Value 0 means not to retry at
+	all; -1 means to try indefinitely. Default is 100 (i.e.,
+	retry for 100ms).
+
 core.packedRefsTimeout::
 	The length of time, in milliseconds, to retry when trying to
 	lock the `packed-refs` file. Value 0 means not to retry at