add: speed up cmd_add() by utilizing read_cache_preload()

During an "add", a call is made to run_diff_files() which calls
check_removed() for each index-entry.  The preload_index() code
distributes some of the costs across multiple threads.

Because the files checked are restricted to pathspec, adding
individual files makes no measurable impact but on a Windows repo
with ~200K files, 'git add .' drops from 6.3 seconds to 3.3 seconds
for a 47% savings.

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: benpeart

builtin/add.c

@@ -445,11 +445,6 @@ int cmd_add(int argc, const char **argv, const char *prefix)
 		return 0;
 	}
 
-	if (read_cache() < 0)
-		die(_("index file corrupt"));
-
-	die_in_unpopulated_submodule(&the_index, prefix);
-
 	/*
 	 * Check the "pathspec '%s' did not match any files" block
 	 * below before enabling new magic.
@@ -459,6 +454,10 @@ int cmd_add(int argc, const char **argv, const char *prefix)
 		       PATHSPEC_SYMLINK_LEADING_PATH,
 		       prefix, argv);
 
+	if (read_cache_preload(&pathspec) < 0)
+		die(_("index file corrupt"));
+
+	die_in_unpopulated_submodule(&the_index, prefix);
 	die_path_inside_submodule(&the_index, &pathspec);
 
 	if (add_new_files) {