merge-recursive: ensure full index

Before iterating over all cache entries, ensure that a sparse index is
expanded to a full index to avoid unexpected behavior.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-recursive.c

@@ -522,6 +522,8 @@ static struct string_list *get_unmerged(struct index_state *istate)
 
 	unmerged->strdup_strings = 1;
 
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(istate);
 	for (i = 0; i < istate->cache_nr; i++) {
 		struct string_list_item *item;
 		struct stage_data *e;