OstreeGpgVerifier: Take the signed data as a GBytes

Similar to c2b01ad.  For some reason I was thinking the commit data
still needed to be written to disk prior to verifying, but it's just
another artifact of spawning gpgv2 (predates using GPGME).

Makes for a nice cleanup in fetch_metadata_to_verify_delta_superblock()
as well.

src/libostree/ostree-repo-private.h

@@ -186,13 +186,13 @@ _ostree_repo_get_remote_boolean_option (OstreeRepo  *self,
                                         GError     **error);
 
 gboolean
-_ostree_repo_gpg_verify_file_with_metadata (OstreeRepo          *self,
-                                            GFile               *path,
-                                            GVariant            *metadata,
-                                            GFile               *keyringdir,
-                                            GFile               *extra_keyring,
-                                            GCancellable        *cancellable,
-                                            GError             **error);
+_ostree_repo_gpg_verify_with_metadata (OstreeRepo          *self,
+                                       GBytes              *signed_data,
+                                       GVariant            *metadata,
+                                       GFile               *keyringdir,
+                                       GFile               *extra_keyring,
+                                       GCancellable        *cancellable,
+                                       GError             **error);
 
 gboolean
 _ostree_repo_commit_loose_final (OstreeRepo        *self,