git prompt: reindent

Normally I wouldn't bother, but it was difficult to follow the nested
if statements.  Most of the file used tabs, so replaced all leading
spaces with the appropriate amount of tabs.
git prompt: actually show staged token
git prompt: replace question with explanation

The code in question displays GIT_DIR! if the user is inside the .git
directory of a repository that has a working directory.  Several git
commands won't work in that situation, so it's useful to warn the
user.
Added completions for apt-mark