worktree remove: allow it when $GIT_WORK_TREE is already gone

"git worktree remove" basically consists of two things

- delete $GIT_WORK_TREE
- delete $GIT_DIR (which is $SUPER_GIT_DIR/worktrees/something)

If $GIT_WORK_TREE is already gone for some reason, we should be able
to finish the job by deleting $GIT_DIR.

Two notes:

- $GIT_WORK_TREE _can_ be missing if the worktree is locked. In that
  case we must not delete $GIT_DIR because the real $GIT_WORK_TREE may
  be in a usb stick somewhere. This is already handled because we
  check for lock first.

- validate_worktree() is still called because it may do more checks in
  future (and it already does something else, like checking main
  worktree, but that's irrelevant in this case)

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

worktree.c

@@ -267,7 +267,8 @@ static void strbuf_addf_gently(struct strbuf *buf, const char *fmt, ...)
 	va_end(params);
 }
 
-int validate_worktree(const struct worktree *wt, struct strbuf *errmsg)
+int validate_worktree(const struct worktree *wt, struct strbuf *errmsg,
+		      unsigned flags)
 {
 	struct strbuf wt_path = STRBUF_INIT;
 	char *path = NULL;
@@ -303,6 +304,12 @@ int validate_worktree(const struct worktree *wt, struct strbuf *errmsg)
 		goto done;
 	}
 
+	if (flags & WT_VALIDATE_WORKTREE_MISSING_OK &&
+	    !file_exists(wt->path)) {
+		ret = 0;
+		goto done;
+	}
+
 	if (!file_exists(wt_path.buf)) {
 		strbuf_addf_gently(errmsg, _("'%s' does not exist"), wt_path.buf);
 		goto done;