status: only touch path we may need to check

This patch gets rid of whole-tree cache refresh and untracked file
search. Instead only specified path will be looked at.

Again some numbers on gentoo-x86, ~80k files:

Unmodified Git:

$ time git st eclass/
nothing to commit (working directory clean)

real    0m3.211s
user    0m1.977s
sys     0m1.135s

Modified Git:

$ time ~/w/git/git st eclass/
nothing to commit (working directory clean)

real    0m1.587s
user    0m1.426s
sys     0m0.111s

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-commit.c

@@ -1023,7 +1023,7 @@ int cmd_status(int argc, const char **argv, const char *prefix)
 		s.pathspec = get_pathspec(prefix, argv);
 
 	read_cache();
-	refresh_cache(REFRESH_QUIET|REFRESH_UNMERGED);
+	refresh_index(&the_index, REFRESH_QUIET|REFRESH_UNMERGED, s.pathspec, NULL, NULL);
 	s.is_initial = get_sha1(s.reference, sha1) ? 1 : 0;
 	s.in_merge = in_merge;
 	wt_status_collect(&s);

wt-status.c

@@ -343,7 +343,7 @@ static void wt_status_collect_untracked(struct wt_status *s)
 			DIR_SHOW_OTHER_DIRECTORIES | DIR_HIDE_EMPTY_DIRECTORIES;
 	setup_standard_excludes(&dir);
 
-	fill_directory(&dir, NULL);
+	fill_directory(&dir, s->pathspec);
 	for (i = 0; i < dir.nr; i++) {
 		struct dir_entry *ent = dir.entries[i];
 		if (!cache_name_is_other(ent->name, ent->len))