Add diffstat width config option

The diffstat --stat-width and --stat-namewidth options are nice, but do not affect the width for merges (which would be nice). I will shortly send three patches that add config options 'diff.stat.width' and 'diff.stat.namewidth'.

I haven't written C code in a long time, and hopefully I didn't introduce any bugs or deviate too far from the established Git source code style. I look forward to any comments.

Thank you,
Matt Ruffalo...