Escape slashes in the branch name before passing it to `mktemp`.
Otherwise it would complain with:
mktemp: invalid suffix [...], contains directory separator
Add description in one sentence and use cases for this.
Remove sentence about removing `TEMP_DIR` that is outdated since:
5a8b5ffd21 ("git:checkout-worktree: Use `mktemp` template")
Use a template that directly includes the repository name and the branch
checked out. This makes finding and deleting the worktree directory if
anything goes wrong a lot easier.
Checkout the worktree directly in the temporary directory. With that
`TEMP_DIR` becomes obsolete.
Fix the exit condition when the function is called outside of a git
repository.
Use zsh's `:t` History Expansion Modifier, as `$?` does not keep `git
rev-parse`'s exit code, but rather `basename`'s.
