I was looking at https://bugzilla.gnome.org/show_bug.cgi?id=738954
which wants us to ensure we chown() the refs. As part of that,
I did a generic conversion to use `*at()` (which naturally gives
us more low level control so we can call `fchown` etc.
This patch also sneaks in a change to respect the repo's
`disable_fsync` flag - if fsync is not set, then we never
`fdatasync()` (unlike the `g_file_replace_contents()` default. Also
unlike it, if fsync is enabled, we *always* sync even if the file
didn't exist.
Starting down the path of not using libgsystem. The main win here
will be code sharing between ostree/rpm-ostree as well as going down
the path of not using GFile * for local files.
Matthew Barnes
Colin Walters
Daniel Svensson