reset [--mixed]: use diff-based reset whether or not pathspec was given

Thanks to b65982b (Optimize "diff-index --cached" using cache-tree,
2009-05-20), resetting with paths is much faster than resetting
without paths. Some timings for the linux-2.6 repo to illustrate this
(best of five, warm cache):

        reset       reset .
real    0m0.219s    0m0.080s
user    0m0.140s    0m0.040s
sys     0m0.070s    0m0.030s

These two commands should do the same thing, so instead of having the
user type the trailing " ." to get the faster do_diff_cache()-based
implementation, always use it when doing a mixed reset, with or
without paths (so "git reset $rev" would also be faster).

Timing "git reset" shows that it indeed becomes as fast as
"git reset ." after this patch.

Signed-off-by: Martin von Zweigbergk <martinvonz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/reset.c

@@ -322,7 +322,7 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 	if (reset_type != SOFT) {
 		struct lock_file *lock = xcalloc(1, sizeof(struct lock_file));
 		int newfd = hold_locked_index(lock, 1);
-		if (pathspec) {
+		if (reset_type == MIXED) {
 			if (read_from_tree(pathspec, sha1))
 				return 1;
 		} else {