Merge branch 'nd/clone-linked-checkout' into maint

Merge branch 'nd/clone-linked-checkout' into maint

It was not possible to use a repository-lookalike created by "git
worktree add" as a local source of "git clone".

* nd/clone-linked-checkout:
  clone: better error when --reference is a linked checkout
  clone: allow --local from a linked checkout
  enter_repo: allow .git files in strict mode
  enter_repo: avoid duplicating logic, use is_git_directory() instead
  t0002: add test for enter_repo(), non-strict mode
  path.c: delete an extra space