worktree.c: add is_main_worktree()

Main worktree _is_ different. You can lock (*) a linked worktree but not
the main one, for example. Provide an API for checking that.

(*) Add the file $GIT_DIR/worktrees/xxx/locked to avoid worktree xxx
from being removed or moved.

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

worktree.h

@@ -37,6 +37,11 @@ extern struct worktree *find_worktree(struct worktree **list,
 				      const char *prefix,
 				      const char *arg);
 
+/*
+ * Return true if the given worktree is the main one.
+ */
+extern int is_main_worktree(const struct worktree *wt);
+
 /*
  * Free up the memory for worktree(s)
  */