Fix double "close()" in ce_compare_data

[Linus Torvalds <torvalds@osdl.org>]

Doing an "strace" on "git diff" shows that we close() a file descriptor 
twice (getting EBADFD on the second one) when we end up in ce_compare_data 
if the index does not match the checked-out stat information.

The "index_fd()" function will already have closed the fd for us, so we 
should not close it again.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
---

The way I found this also showed a potential performance problem: if you 
do a "git reset --hard" (or similar) after you have changes in your tree, 
it will write the index file with the same timestamp as the checked out 
files that it re-wrote.

That will also then forever afterwards (well, until the next "git 
update-index --refresh") cause the "uncommon" timestamp case in 
ce_match_stat(), where we check the index-file timestamp against the 
timestamp of the stat data, to trigger.

Not very good. The "ce_modified_check_fs()" tests can be quite expensive 
if you have lots of those files because we end up then calling the 
"ce_compare_data()" function a lot. And suddenly "git diff" doesn't take a 
tenth of a second any more.

We should really try to have some way to re-generate the index 
automatically when this case triggers, so that we only need to do it 
_once_ rather than keep doing it forever while the index is "potentially 
stale".

Any ideas?

diff --git a/read-cache.c b/read-cache.c
index c0b0313..f92cdaa 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -61,7 +61,7 @@ static int ce_compare_data(struct cache_
 		unsigned char sha1[20];
 		if (!index_fd(sha1, fd, st, 0, NULL))
 			match = memcmp(sha1, ce->sha1, 20);
-		close(fd);
+		/* index_fd() closed the file descriptor already */
 	}
 	return match;
 }