A couple bug fixes

I moved the fringe bitmap initialization into the minor mode definition. As it was, the fringe bitmaps were not being loaded unless the turn-on-diff-hl-mode function was used (or the global minor mode function), rather than invoking the minor mode function directly.

Also, in turn-on-diff-hl-mode, the all initialization code was inside
(when (window-sytem) ...)
preventing the mode from being enabled at all in a terminal (and not just
the fringe bitmaps from being enabled).