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