write_index(): update index_state->timestamp after flushing to disk

Since this timestamp is used to check for racy-clean files, it is
important to keep it uptodate.

For the 'git checkout' command without the '-q' option, this make a
huge difference.  Before, each and every file which was updated, was
racy-clean after the call to unpack_trees() and write_index() but
before the GIT process ended.

And because of the call to show_local_changes() in builtin-checkout.c,
we ended up reading those files back into memory, doing a SHA1 to
check if the files was really different from the index.  And, of
course, no file was different.

With this fix, 'git checkout' without the '-q' option should now be
almost as fast as with the '-q' option, but not quite, as we still do
some few lstat(2) calls more without the '-q' option.

Below is some average numbers for 10 checkout's to v2.6.27 and 10 to
v2.6.25 of the Linux kernel, to show the difference:

before (git version 1.6.2.rc1.256.g58a87):
 7.860 user  2.427 sys  19.465 real  52.8% CPU  faults: 0 major 95331 minor
after:
 6.184 user  2.160 sys  17.619 real  47.4% CPU  faults: 0 major 38994 minor

Signed-off-by: Kjetil Barvik <barvik@broadpark.no>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -430,7 +430,7 @@ extern int read_index_preload(struct index_state *, const char **pathspec);
 extern int read_index_from(struct index_state *, const char *path);
 extern int is_index_unborn(struct index_state *);
 extern int read_index_unmerged(struct index_state *);
-extern int write_index(const struct index_state *, int newfd);
+extern int write_index(struct index_state *, int newfd);
 extern int discard_index(struct index_state *);
 extern int unmerged_index(const struct index_state *);
 extern int verify_path(const char *path);

read-cache.c

@@ -1528,13 +1528,14 @@ static int ce_write_entry(git_SHA_CTX *c, int fd, struct cache_entry *ce)
 	return ce_write(c, fd, ondisk, size);
 }
 
-int write_index(const struct index_state *istate, int newfd)
+int write_index(struct index_state *istate, int newfd)
 {
 	git_SHA_CTX c;
 	struct cache_header hdr;
 	int i, err, removed, extended;
 	struct cache_entry **cache = istate->cache;
 	int entries = istate->cache_nr;
+	struct stat st;
 
 	for (i = removed = extended = 0; i < entries; i++) {
 		if (cache[i]->ce_flags & CE_REMOVE)
@@ -1578,7 +1579,14 @@ int write_index(const struct index_state *istate, int newfd)
 		if (err)
 			return -1;
 	}
-	return ce_flush(&c, newfd);
+
+	if (ce_flush(&c, newfd) || fstat(newfd, &st))
+		return -1;
+	istate->timestamp.sec = (unsigned int)st.st_ctime;
+#ifdef USE_NSEC
+	istate->timestamp.nsec = (unsigned int)st.st_ctim.tv_nsec;
+#endif
+	return 0;
 }
 
 /*