worktree.c: add validate_worktree()

This function is later used by "worktree move" and "worktree remove"
to ensure that we have a good connection between the repository and
the worktree. For example, if a worktree is moved manually, the
worktree location recorded in $GIT_DIR/worktrees/.../gitdir is
incorrect and we should not move that one.

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

worktree.h

@@ -3,6 +3,8 @@
 
 #include "refs.h"
 
+struct strbuf;
+
 struct worktree {
 	char *path;
 	char *id;
@@ -59,6 +61,13 @@ extern int is_main_worktree(const struct worktree *wt);
  */
 extern const char *is_worktree_locked(struct worktree *wt);
 
+/*
+ * Return zero if the worktree is in good condition. Error message is
+ * returned if "errmsg" is not NULL.
+ */
+extern int validate_worktree(const struct worktree *wt,
+			     struct strbuf *errmsg);
+
 /*
  * Free up the memory for worktree(s)
  */