Support force writing reflog on a per-update basis

Support force writing reflog on a per-update basis

Even if a repository has core.logAllRefUpdates=true, ReflogWriter does
not create reflog files unless the refs are under a hard-coded list of
prefixes, or unless the forceWrite bit is set. Expose the forceWrite bit
on a per-update basis in RefUpdate/BatchRefUpdate/ReceiveCommand,
creating RefLogWriters as necessary.

Change-Id: Ifc851fba00f76bf56d4134f821d0576b37810f80