branch: update output to include worktree info

branch: update output to include worktree info

The output of git branch is modified to mark branches checked out in a
linked worktree with a "+" and color them in cyan (in contrast to the
current branch, which will still be denoted with a "*" and colored in green)

This is meant to communicate to the user that the branches that are
marked or colored will behave differently from other branches if the user
attempts to check them out or delete them, since branches checked out in
another worktree cannot be checked out or deleted.

Signed-off-by: Nickolai Belakovski <nbelakovski@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>