Change OstreeFetcher to be dirfd-relative

This is a noticeable cleanup, and fixes another big user of GFile* in
performance/security sensitive codepaths.

I'm specifically making this change because the static deltas code was
leaking temporary files, and cleaning that up nicely would be best if
we were fd relative.

src/libostree/ostree-metalink.h

@@ -53,7 +53,7 @@ SoupURI *_ostree_metalink_get_uri (OstreeMetalink         *self);
 gboolean _ostree_metalink_request_sync (OstreeMetalink        *self,
                                         GMainLoop             *loop,
                                         SoupURI               **out_target_uri,
-                                        GFile                 **out_data,
+                                        char                  **out_data,
                                         SoupURI               **fetching_sync_uri,
                                         GCancellable          *cancellable,
                                         GError                **error);