diffstat: made test case work with POSIX sh and printf
* arithmetic expression ((...)), without $, is bashism.
* printf '\xXX' seems non-standard. '\0' is okay.
http://www.opengroup.org/onlinepubs/
009695399/utilities/printf.html
tested with bash 4.0 and dash 0.5.5.1
diffstat: with --git, mark binary files with Bin
Normally, diffs without any text insertions or deletions are reported
as having 0 lines changed by stock diffstat. Compatibility is
preserved with stock diffstat in this case, but when using --git,
binary files are marked with Bin as a means of clarification.
git diff --stat does something similar, though it also includes the
old and new file sizes.