fsmonitor: change last update timestamp on the index_state to opaque token

Some file system monitors might not use or take a timestamp for processing
and in the case of watchman could have race conditions with using a
timestamp. Watchman uses something called a clockid that is used for race
free queries to it. The clockid for watchman is simply a string.

Change the fsmonitor_last_update from being a uint64_t to a char pointer
so that any arbitrary data can be stored in it and passed back to the
fsmonitor.

Signed-off-by: Kevin Willford <Kevin.Willford@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/cache.h b/cache.h
index cbfaead..1a2e1cc 100644
--- a/cache.h
+++ b/cache.h
@@ -324,7 +324,7 @@ struct index_state {
 	struct hashmap dir_hash;
 	struct object_id oid;
 	struct untracked_cache *untracked;
-	uint64_t fsmonitor_last_update;
+	char *fsmonitor_last_update;
 	struct ewah_bitmap *fsmonitor_dirty;
 	struct mem_pool *ce_mem_pool;
 	struct progress *progress;