commit: skip discarding the index if there is no pre-commit hook

If there is not a pre-commit hook, there is no reason to discard
the index and reread it.

This change checks to presence of a pre-commit hook and then only
discards the index if there was one.

Signed-off-by: Kevin Willford <kewillf@microsoft.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit.c

@@ -943,13 +943,16 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 		return 0;
 	}
 
-	/*
-	 * Re-read the index as pre-commit hook could have updated it,
-	 * and write it out as a tree.  We must do this before we invoke
-	 * the editor and after we invoke run_status above.
-	 */
-	discard_cache();
+	if (!no_verify && find_hook("pre-commit")) {
+		/*
+		 * Re-read the index as pre-commit hook could have updated it,
+		 * and write it out as a tree.  We must do this before we invoke
+		 * the editor and after we invoke run_status above.
+		 */
+		discard_cache();
+	}
 	read_cache_from(index_file);
+
 	if (update_main_cache_tree(0)) {
 		error(_("Error building trees"));
 		return 0;