libglnx porting: Migrate to new tempfile code

In general this is even cleaner now, though it was better after I
extracted a helper function for the "write tempfile with contents"
bits that were shared between metadata and regular file codepaths.

Closes: #369
Approved by: jlebon

src/libostree/ostree-repo-private.h

@@ -255,6 +255,7 @@ _ostree_repo_commit_loose_final (OstreeRepo        *self,
                                  const char        *checksum,
                                  OstreeObjectType   objtype,
                                  int                temp_dfd,
+                                 int                fd,
                                  const char        *temp_filename,
                                  GCancellable      *cancellable,
                                  GError           **error);