path.c: move some code out of strbuf_git_path_submodule()

refs is learning to avoid path rewriting that is done by
strbuf_git_path_submodule(). Factor out this code so it could be reused
by refs_* functions.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule.h

@@ -81,6 +81,12 @@ extern int push_unpushed_submodules(struct sha1_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);
 
 /*
  * Prepare the "env_array" parameter of a "struct child_process" for executing