Re: [PATCH 22/32] checkout: support checking out into a new working directory

> MULTIPLE WORKING TREES
> ----------------------
> 
> A git repository can support multiple working trees, allowing you to check
> out more than one branch at a time.  With `git checkout --to` a new working
> tree is associated with the repository.  This new working tree is called a
> "linked working tree" as opposed to the "main working tree" prepared by "git
> init" or "git clone".  A repository has one main working tree (if it's not a
> bare repository) and zero or more linked working trees.
> 
> Each linked working tree has a private sub-directory in the repository's
> $GIT_DIR/worktrees directory.  The private sub-directory's name is usually
> the base name of the linked working tree's path, possibly appended with a
> number to make it unique.  For example, when `$GIT_DIR=/path/main/.git` the
> command `git checkout --to /path/other/test-next next` creates the linked
> working tree in `/path/other/test-next` and also creates a
> `$GIT_DIR/worktrees/test-next` directory (or `$GIT_DIR/worktrees/test-next1`
> if `test-next` is already taken).