To make integrating with plugin managers as easy as possible,
plugins/scripts shouldn't be sourced on their own; instead, they should
export functions.
Split former `parse_git_status` function, which was around 60 lines of
code, into multiple small functions.
Also, instead of polluting the global namespace with a lot of variables,
make functions return something through a $REPLY variable. Or the
$STATUS variable, in the case of the *new* parse_git_status function.
