worktree.c: store "id" instead of "git_dir"

We can reconstruct git_dir from id quite easily. It's a bit hackier to
do the reverse.

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

worktree.h

@@ -3,7 +3,7 @@
 
 struct worktree {
 	char *path;
-	char *git_dir;
+	char *id;
 	char *head_ref;
 	unsigned char head_sha1[20];
 	int is_detached;
@@ -22,6 +22,12 @@ struct worktree {
  */
 extern struct worktree **get_worktrees(void);
 
+/*
+ * Return git dir of the worktree. Note that the path may be relative.
+ * If wt is NULL, git dir of current worktree is returned.
+ */
+extern const char *get_worktree_git_dir(const struct worktree *wt);
+
 /*
  * Free up the memory for worktree(s)
  */