squash! add: use preload-index and fscache for performance

We need to keep the call to read_cache() before parsing the
pathspecs (and hence cannot use the pathspecs to limit any preload)
because parse_pathspec() is using the index to determine whether a
pathspec is, in fact, in a submodule. If we would not read the index
first, parse_pathspec() would not error out on a path that is inside
a submodule, and t7400-submodule-basic.sh would fail with

	not ok 47 - do not add files from a submodule

We still want the nice preload performance boost, though, so we simply
call read_cache_preload(&pathspecs) after parsing the pathspecs.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

builtin/add.c

@@ -376,6 +376,9 @@ int cmd_add(int argc, const char **argv, const char *prefix)
 		return 0;
 	}
 
+	if (read_cache() < 0)
+		die(_("index file corrupt"));
+
 	/*
 	 * Check the "pathspec '%s' did not match any files" block
 	 * below before enabling new magic.
@@ -387,8 +390,8 @@ int cmd_add(int argc, const char **argv, const char *prefix)
 		       prefix, argv);
 
 	enable_fscache(1);
-	if (read_cache_preload(&pathspec) < 0)
-		die(_("index file corrupt"));
+	/* We do not really re-read the index, but update the up-to-date flags */
+	preload_index(&the_index, &pathspec);
 
 	if (add_new_files) {
 		int baselen;