read-cache: free threaded memory pool

In load_cache_entries_threaded(), each thread allocates its own memory
pool. This pool needs to be cleaned up while closing the threads down,
or it will be leaked.

This ce_mem_pool pointer could theoretically be converted to an inline
copy of the struct, but the use of a pointer helps with existing lazy-
initialization logic. Adjusting that behavior only to avoid this pointer
would be a much bigger change.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

read-cache.c

@@ -2187,6 +2187,7 @@ static unsigned long load_cache_entries_threaded(struct index_state *istate, con
 		if (err)
 			die(_("unable to join load_cache_entries thread: %s"), strerror(err));
 		mem_pool_combine(istate->ce_mem_pool, p->ce_mem_pool);
+		free(p->ce_mem_pool);
 		consumed += p->consumed;
 	}