unpack-trees: populate cache-tree on successful merge

When we unpack trees into an existing index, we discard the old
index and replace it with the new, merged index.  Ensure that this
index has its cache-tree populated.  This will make subsequent git
status and commit commands faster.

Signed-off-by: Brian Degenhardt <bmd@bmdhacks.com>
Signed-off-by: David Turner <dturner@twopensource.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0090-cache-tree.sh

@@ -199,6 +199,30 @@ test_expect_success 'checkout -B gives cache-tree' '
 	test_cache_tree
 '
 
+test_expect_success 'merge --ff-only maintains cache-tree' '
+	git checkout current &&
+	git checkout -b changes &&
+	test_commit llamas &&
+	test_commit pachyderm &&
+	test_cache_tree &&
+	git checkout current &&
+	test_cache_tree &&
+	git merge --ff-only changes &&
+	test_cache_tree
+'
+
+test_expect_success 'merge maintains cache-tree' '
+	git checkout current &&
+	git checkout -b changes2 &&
+	test_commit alpacas &&
+	test_cache_tree &&
+	git checkout current &&
+	test_commit struthio &&
+	test_cache_tree &&
+	git merge changes2 &&
+	test_cache_tree
+'
+
 test_expect_success 'partial commit gives cache-tree' '
 	git checkout -b partial no-children &&
 	test_commit one &&

unpack-trees.c

@@ -1155,6 +1155,14 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	o->src_index = NULL;
 	ret = check_updates(o) ? (-2) : 0;
 	if (o->dst_index) {
+		if (!ret) {
+			if (!o->result.cache_tree)
+				o->result.cache_tree = cache_tree();
+			if (!cache_tree_fully_valid(o->result.cache_tree))
+				cache_tree_update(&o->result,
+						  WRITE_TREE_SILENT |
+						  WRITE_TREE_REPAIR);
+		}
 		discard_index(o->dst_index);
 		*o->dst_index = o->result;
 	} else {