Currently, the user experience when creating worktrees that track remote
upstreams are not straight forward. This patch improves it by allowing the
user to pass any ref to the --track option of git.

This is especially useful when creating or deleting a worktree, to avoid
deleting errors if the user is within the to-be deleted worktree or
unintended nested paths when creating.

Follow the same naming convention of the main picker.

This means that a user can create a worktree when there is none, so allow
for an empty worktree list by not returning early if #results == 0.

Other pickers already use <c-d> for scrolling preview and
telescope-file-browser.nvim use <m-d> for deleting files, so use that for
consistency.

This corrects the output and make unnecessary to make use of :gsub and
:trim.

Currently, if the user chooses a remote branch to checkout, it will be
created locally with the same name causing it to produce an ambiguous ref
which git won't be able to use as upstream to track. This defaults the new
local branch to be prefixed with "local/".