magit-wash-diffstat: split regex to make it easier on humans

This is just in preparation to make it longer.
magit-wash-diffstat: support binary files

Binary files are displayed with "Bin XXX -> YYY bytes" instead of the
change count and graph. The numbers correspond to the old and new sizes
of the binary file, not lines added/removed, hence the capture groups
for these are left empty and this text left with no special faces.
Merge pull request #1253 from afrepues/fix/truncated-diffstat-with-binary-files

Fix truncated diffstat with binary files