fsmonitor: do not forget to release the token in `discard_index()`

In 56c6910028 (fsmonitor: change last update timestamp on the
index_state to opaque token, 2020-01-07), we forgot to adjust
`discard_index()` to release the "last-update" token: it is no longer a
64-bit number, but a free-form string that has been allocated.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

read-cache.c

@@ -2355,6 +2355,7 @@ int discard_index(struct index_state *istate)
 	cache_tree_free(&(istate->cache_tree));
 	istate->initialized = 0;
 	istate->fsmonitor_has_run_once = 0;
+	FREE_AND_NULL(istate->fsmonitor_last_update);
 	FREE_AND_NULL(istate->cache);
 	istate->cache_alloc = 0;
 	discard_split_index(istate);