Merge branch 'tg/worktree-create-tracking'

The way "git worktree add" determines what branch to create from
where and checkout in the new worktree has been updated a bit.

* tg/worktree-create-tracking:
  add worktree.guessRemote config option
  worktree: add --guess-remote flag to add subcommand
  worktree: make add <path> <branch> dwim
  worktree: add --[no-]track option to the add subcommand
  worktree: add can be created from any commit-ish
  checkout: factor out functions to new lib file

diff --cc Documentation/config.txt
Simple merge

diff --cc Makefile
Simple merge

diff --cc builtin/checkout.c
Simple merge