repo: After renaming in all loose objects, ensure metadata is stable

repo: After renaming in all loose objects, ensure metadata is stable

When a transaction is finished and we have moved all the staged loose
objects into the repo we fsync all the object directory, to ensure the
filenames are stable before we update the refs files to point to the
new commits.

With out this an unclean shutdown after the transaction is finished
could result in a refs file that points to an incomplete commit.

https://bugzilla.gnome.org/show_bug.cgi?id=759442

Closes: #918
Approved by: cgwalters