Skip submodule head/index update when caching.

`git_submodule_status` is very slow, bottlenecked on
`git_repository_head_tree`, which it uses through `submodule_update_head`.  If
the user has requested submodule caching, assume that they want this status
cached too and skip it.

Signed-off-by: David Turner <dturner@twosigma.com>

src/submodule.c

@@ -1544,6 +1544,14 @@ int git_submodule__status(
 		return 0;
 	}
 
+	/* If the user has requested caching submodule state, performing these
+	 * expensive operations (especially `submodule_update_head`, which is
+	 * bottlenecked on `git_repository_head_tree`) eliminates much of the
+	 * advantage.  We will, therefore, interpret the request for caching to
+	 * apply here to and skip them.
+	 */
+
+	if (sm->repo->submodule_cache == NULL) {
 		/* refresh the index OID */
 		if (submodule_update_index(sm) < 0)
 			return -1;
@@ -1551,6 +1559,7 @@ int git_submodule__status(
 		/* refresh the HEAD OID */
 		if (submodule_update_head(sm) < 0)
 			return -1;
+	}
 
 	/* for ignore == dirty, don't scan the working directory */
 	if (ign == GIT_SUBMODULE_IGNORE_DIRTY) {