Merge branch 'cw/worktree-extension'

Introduce a new repository extension to prevent older Git versions
from mis-interpreting worktrees created with relative paths.

* cw/worktree-extension:
  worktree: refactor `repair_worktree_after_gitdir_move()`
  worktree: add relative cli/config options to `repair` command
  worktree: add relative cli/config options to `move` command
  worktree: add relative cli/config options to `add` command
  worktree: add `write_worktree_linking_files()` function
  worktree: refactor infer_backlink return
  worktree: add `relativeWorktrees` extension
  setup: correctly reinitialize repository version

diff --cc builtin/worktree.c
Simple merge

diff --cc t/t0001-init.sh
Simple merge

diff --cc t/t2400-worktree-add.sh
Simple merge

diff --cc t/t2401-worktree-prune.sh
Simple merge

diff --cc t/t2402-worktree-list.sh
Simple merge

diff --cc t/t2403-worktree-move.sh
Simple merge

diff --cc t/t2406-worktree-repair.sh
Simple merge

diff --cc t/t5504-fetch-receive-strict.sh
Simple merge