[RFC] 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...