unpack-trees: ensure full index

The next change will translate full indexes into sparse indexes at write
time. The existing logic provides a way for every sparse index to be
expanded to a full index at read time. However, there are cases where an
index is written and then continues to be used in-memory to perform
further updates.

unpack_trees() is frequently called after such a write. In particular,
commands like 'git reset' do this double-update of the index.

Ensure that we have a full index when entering unpack_trees(), but only
when command_requires_full_index is true. This is always true at the
moment, but we will later relax that after unpack_trees() is updated to
handle sparse directory entries.

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

unpack-trees.c

@@ -1567,6 +1567,7 @@ static int verify_absent(const struct cache_entry *,
  */
 int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options *o)
 {
+	struct repository *repo = the_repository;
 	int i, ret;
 	static struct cache_entry *dfc;
 	struct pattern_list pl;
@@ -1578,6 +1579,12 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	trace_performance_enter();
 	trace2_region_enter("unpack_trees", "unpack_trees", the_repository);
 
+	prepare_repo_settings(repo);
+	if (repo->settings.command_requires_full_index) {
+		ensure_full_index(o->src_index);
+		ensure_full_index(o->dst_index);
+	}
+
 	if (!core_apply_sparse_checkout || !o->update)
 		o->skip_sparse_checkout = 1;
 	if (!o->skip_sparse_checkout && !o->pl) {