Add --disable-fsync option to pull-local, and API to repo

This will be used by guestmount - it's WAY faster.  We only take disks
as a unit, so it's safe.  If the process fails halfway through, we
just start over from scratch the next time anyways.

src/libostree/ostree-repo.h

@@ -48,6 +48,9 @@ gboolean      ostree_repo_open   (OstreeRepo     *self,
                                   GCancellable   *cancellable,
                                   GError        **error);
 
+void          ostree_repo_set_disable_fsync (OstreeRepo    *self,
+                                             gboolean       disable_fsync);
+
 gboolean      ostree_repo_create (OstreeRepo     *self,
                                   OstreeRepoMode  mode,
                                   GCancellable   *cancellable,