Merge branch 'nd/per-worktree-config'

Merge branch 'nd/per-worktree-config'

A fourth class of configuration files (in addition to the
traditional "system wide", "per user in the $HOME directory" and
"per repository in the $GIT_DIR/config") has been introduced so
that different worktrees that share the same repository (hence the
same $GIT_DIR/config file) can use different customization.

* nd/per-worktree-config:
  worktree: add per-worktree config files
  t1300: extract and use test_cmp_config()