update $GIT_INDEX_FILE when there are racily clean entries

Traditional "opportunistic index update" done by read-only "diff" and
"status" was about updating cached lstat(2) information in the index for
the next round.  We missed another obvious optimization opportunity: when
there are racily clean entries that will cease to be racily clean by
updating $GIT_INDEX_FILE.  Detect that case and write $GIT_INDEX_FILE out
to give it a newer timestamp.

Noticed by Lasse Makholm by stracing "git status" in a fresh checkout and
counting the number of open(2) calls.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

read-cache.c

diff --git a/read-cache.c b/read-cache.c
index 7a0421c..971e277 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1545,12 +1545,25 @@
 	return result;
 }
 
+static int has_racy_timestamp(struct index_state *istate)
+{
+	int entries = istate->cache_nr;
+	int i;
+
+	for (i = 0; i < entries; i++) {
+		struct cache_entry *ce = istate->cache[i];
+		if (is_racy_timestamp(istate, ce))
+			return 1;
+	}
+	return 0;
+}
+
 /*
  * Opportunisticly update the index but do not complain if we can't
  */
 void update_index_if_able(struct index_state *istate, struct lock_file *lockfile)
 {
-	if (istate->cache_changed &&
+	if ((istate->cache_changed || has_racy_timestamp(istate)) &&
 	    !write_index(istate, lockfile->fd))
 		commit_locked_index(lockfile);
 	else