Fix "git commit directory/" performance anomaly

This trivial patch avoids re-hashing files that are already clean in the
index. This mirrors what commit 0781b8a9
did for "git add .", only for "git commit ." instead.

This improves the cold-cache case immensely, since we don't need to bring
in all the file contents, just the index and any files dirty in the index.

Before:

	[torvalds@woody linux]$ time git commit .
	real    1m49.537s
	user    0m3.892s
	sys     0m2.432s

After:

	[torvalds@woody linux]$ time git commit .
	real    0m14.273s
	user    0m1.312s
	sys     0m0.516s

(both after doing a "echo 3 > /proc/sys/vm/drop_caches" to get cold-cache
behaviour - even with the index optimization git still has to "lstat()"
all the files, so with a truly cold cache, bringing all the inodes in
will take some time).

[jc: trivial "return 0;" fixed]
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-update-index.c

@@ -86,9 +86,15 @@ static int process_lstat_error(const char *path, int err)
 
 static int add_one_path(struct cache_entry *old, const char *path, int len, struct stat *st)
 {
-	int option, size = cache_entry_size(len);
-	struct cache_entry *ce = xcalloc(1, size);
+	int option, size;
+	struct cache_entry *ce;
+
+	/* Was the old index entry already up-to-date? */
+	if (old && !ce_stage(old) && !ce_match_stat(old, st, 0))
+		return 0;
 
+	size = cache_entry_size(len);
+	ce = xcalloc(1, size);
 	memcpy(ce->name, path, len);
 	ce->ce_flags = htons(len);
 	fill_stat_cache_info(ce, st);