repo: workaround empty --show-prefix output from certain git versions
argv: separate git log parameters from diff parameters

Parse of git log parameters and ensure they end up in %(revargs) rather
than %(cmdlineargs). This fixes use cases where tig is opened with some
search (e.g. `tig -SREQ_SCROLL_PAGE_UP`) and the diff view is opened
either from the main view or for example via the blame view.
options: enable graph when --graph is passed