cache-tree: allow writing in-memory index as tree

The function `write_in_core_index_as_tree()` takes a repository and
writes its index into a tree object. What this function cannot do though
is to take an _arbitrary_ in-memory index.

Introduce a new `struct index_state` parameter so that the caller can
pass a different index than the one belonging to the repository. This
will be used in a subsequent commit.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

builtin/checkout.c

@@ -891,7 +891,8 @@ static int merge_working_tree(const struct checkout_opts *opts,
 					   0);
 			init_ui_merge_options(&o, the_repository);
 			o.verbosity = 0;
-			work = write_in_core_index_as_tree(the_repository);
+			work = write_in_core_index_as_tree(the_repository,
+							   the_repository->index);
 
 			ret = reset_tree(new_tree,
 					 opts, 1,

cache-tree.c

@@ -723,11 +723,11 @@ static int write_index_as_tree_internal(struct object_id *oid,
 	return 0;
 }
 
-struct tree* write_in_core_index_as_tree(struct repository *repo) {
+struct tree *write_in_core_index_as_tree(struct repository *repo,
+					 struct index_state *index_state) {
 	struct object_id o;
 	int was_valid, ret;
 
-	struct index_state *index_state	= repo->index;
 	was_valid = index_state->cache_tree &&
 		    cache_tree_fully_valid(index_state->cache_tree);
 

cache-tree.h

@@ -47,7 +47,8 @@ int cache_tree_verify(struct repository *, struct index_state *);
 #define WRITE_TREE_UNMERGED_INDEX (-2)
 #define WRITE_TREE_PREFIX_ERROR (-3)
 
-struct tree* write_in_core_index_as_tree(struct repository *repo);
+struct tree *write_in_core_index_as_tree(struct repository *repo,
+					 struct index_state *index_state);
 int write_index_as_tree(struct object_id *oid, struct index_state *index_state, const char *index_path, int flags, const char *prefix);
 void prime_cache_tree(struct repository *, struct index_state *, struct tree *);