Merge branch 'dt/unpack-trees-cache-tree-revalidate'

The code to perform multi-tree merges has been taught to repopulate
the cache-tree upon a successful merge into the index, so that
subsequent "diff-index --cached" (hence "status") and "write-tree"
(hence "commit") will go faster.

The same logic in "git checkout" may now be removed, but that is a
separate issue.

* dt/unpack-trees-cache-tree-revalidate:
  unpack-trees: populate cache-tree on successful merge

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

@@ -1160,6 +1160,14 @@ 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 {