Merge branch 'nd/files-backend-git-dir'

The "submodule" specific field in the ref_store structure is
replaced with a more generic "gitdir" that can later be used also
when dealing with ref_store that represents the set of refs visible
from the other worktrees.

* nd/files-backend-git-dir: (28 commits)
  refs.h: add a note about sorting order of for_each_ref_*
  t1406: new tests for submodule ref store
  t1405: some basic tests on main ref store
  t/helper: add test-ref-store to test ref-store functions
  refs: delete pack_refs() in favor of refs_pack_refs()
  files-backend: avoid ref api targeting main ref store
  refs: new transaction related ref-store api
  refs: add new ref-store api
  refs: rename get_ref_store() to get_submodule_ref_store() and make it public
  files-backend: replace submodule_allowed check in files_downcast()
  refs: move submodule code out of files-backend.c
  path.c: move some code out of strbuf_git_path_submodule()
  refs.c: make get_main_ref_store() public and use it
  refs.c: kill register_ref_store(), add register_submodule_ref_store()
  refs.c: flatten get_ref_store() a bit
  refs: rename lookup_ref_store() to lookup_submodule_ref_store()
  refs.c: introduce get_main_ref_store()
  files-backend: remove the use of git_path()
  files-backend: add and use files_ref_path()
  files-backend: add and use files_reflog_path()
  ...

submodule.h

@@ -98,6 +98,12 @@ extern int push_unpushed_submodules(struct oid_array *commits,
 				    int dry_run);
 extern void connect_work_tree_and_git_dir(const char *work_tree, const char *git_dir);
 extern int parallel_submodules(void);
+/*
+ * Given a submodule path (as in the index), return the repository
+ * path of that submodule in 'buf'. Return -1 on error or when the
+ * submodule is not initialized.
+ */
+int submodule_to_gitdir(struct strbuf *buf, const char *submodule);
 
 #define SUBMODULE_MOVE_HEAD_DRY_RUN (1<<0)
 #define SUBMODULE_MOVE_HEAD_FORCE   (1<<1)