Core git just looks for NUL bytes in files when deciding about
is-binary inside diff (although it uses a better algorithm in
checkout, when deciding if CRLF conversion should be done).
Libgit2 was using the better algorithm in both places, but that
is causing some confusion.  For now, this makes diff just look
for NUL bytes to decide if a file is binary by content in diff.

There are many scattered functions that look into the contents of
buffers to do various text manipulations (such as escaping or
unescaping data, calculating text stats, guessing if content is
binary, etc).  This groups all those functions together into a
new file and converts the code to use that.

This has two enhancements to existing functionality.  The old
text stats function is significantly rewritten and the BOM
detection code was extended (although largely we can't deal with
anything other than a UTF8 BOM).