Support core.fsyncRefFiles option

If core.fsyncRefFiles is set to true, fsync is used whenever a
reference file is updated, ensuring the file contents are also
written to disk.  This can help to prevent empty ref files after
a system crash when using a filesystem such as HFS+ where data
writes may be delayed.

Change-Id: Ie508a974da50f63b0409c38afe68772322dc19f1
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>