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.
Mark jobs as completed when all processes have finished, not just the last one.
Fixes https://github.com/fish-shell/fish-shell/issues/876 , and coincidentally also https://github.com/fish-shell/fish-shell/issues/848
Merge branch 'master' into ast