git prompt: Fix hang on detached head.

$git_dir was never set in __fish_git_prompt_current_branch, but used
in the case of a detached HEAD.  This caused `cut -c1-7 $git_dir/HEAD`
to expand to `cut -c1-7` which then waited for input.

[Side note:  I find $unset_far/foo expanding to '' instead of '/foo' unexpected]