ce_smudge_racily_clean_entry: explain why it works.

This is a tricky code and warrants extra commenting.  I wasted
30 minutes trying to break it until I realized why it works.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

read-cache.c

@@ -616,7 +616,31 @@ static void ce_smudge_racily_clean_entry(struct cache_entry *ce)
 	if (ce_match_stat_basic(ce, &st))
 		return;
 	if (ce_modified_check_fs(ce, &st)) {
-		/* This is "racily clean"; smudge it */
+		/* This is "racily clean"; smudge it.  Note that this
+		 * is a tricky code.  At first glance, it may appear
+		 * that it can break with this sequence:
+		 *
+		 * $ echo xyzzy >frotz
+		 * $ git-update-index --add frotz
+		 * $ : >frotz
+		 * $ sleep 3
+		 * $ echo filfre >nitfol
+		 * $ git-update-index --add nitfol
+		 *
+		 * but it does not.  Whe the second update-index runs,
+		 * it notices that the entry "frotz" has the same timestamp
+		 * as index, and if we were to smudge it by resetting its
+		 * size to zero here, then the object name recorded
+		 * in index is the 6-byte file but the cached stat information
+		 * becomes zero --- which would then match what we would
+		 * obtain from the filesystem next time we stat("frotz"). 
+		 *
+		 * However, the second update-index, before calling
+		 * this function, notices that the cached size is 6
+		 * bytes and what is on the filesystem is an empty
+		 * file, and never calls us, so the cached size information
+		 * for "frotz" stays 6 which does not match the filesystem.
+		 */
 		ce->ce_size = htonl(0);
 	}
 }