Merge branch 'vd/worktree-config-is-per-repository'

The value of config.worktree is per-repository, but has been kept
in a singleton global variable per process. This has been OK as
most Git operations interacted with a single repository at a time,
but not right for operations like recursive "grep" that want to
access multiple repositories from a single process without forking.

The global variable has been eliminated and made into a member in
the per-repository data structure.

* vd/worktree-config-is-per-repository:
  repository: move 'repository_format_worktree_config' to repo scope
  config: pass 'repo' directly to 'config_with_options()'
  config: use gitdir to get worktree config