worktree.c: add is_worktree_locked()

We need this later to avoid double locking a worktree, or unlocking one
when it's not even locked.

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

worktree.h

@@ -5,10 +5,12 @@ struct worktree {
 	char *path;
 	char *id;
 	char *head_ref;
+	char *lock_reason;	/* internal use */
 	unsigned char head_sha1[20];
 	int is_detached;
 	int is_bare;
 	int is_current;
+	int lock_reason_valid;
 };
 
 /* Functions for acting on the information about worktrees. */
@@ -42,6 +44,12 @@ extern struct worktree *find_worktree(struct worktree **list,
  */
 extern int is_main_worktree(const struct worktree *wt);
 
+/*
+ * Return the reason string if the given worktree is locked or NULL
+ * otherwise.
+ */
+extern const char *is_worktree_locked(struct worktree *wt);
+
 /*
  * Free up the memory for worktree(s)
  */