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()