Fix "git commit directory/" performance anomaly

Fix "git commit directory/" performance anomaly

This trivial patch avoids re-hashing files that are already clean in the
index. This mirrors what commit 0781b8a9b2fe760fc4ed519a3a26e4b9bd6ccffe
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: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>