Fix output of non-UTF8 diff data in python 3
In Python 3, "binary" and "unicode" (i.e. string) data are

distinct types, and only UTF-8 unicode data can be printed

to standard out. In diff.py currently, we attempt to decode

diff data as UTF-8, and we fail if this fails.
Diff data from many SCMs (Git as an example) is not guaranteed

to be UTF-8 data; though this is often a safe assumption, it

is not always the case, and Git will happily output binary

data to the terminal if it does not detect that binary data

as being binary data.
As an example, Chinese language data encoded as Big5 or GB2312

will not be detected as binary data, even though it is also not

UTF-8 data. This means that changing a file encoded in Big5

(in our case, Chinese localization data) makes it impossible

to run rbt diff to visually validate your changes.
In our use case, we also have a workflow of rbt diff to output

to a file, process and validate that diff, and then run rbt post

to submit the diff, so this breaks our workflow in Python 3.
The documentation for Python 3 sys.stdin/out/err notes that

if writing binary data to these streams is required, the stream's

buffer object should be used, so we apply this change when printing

the diff object.
This patch was made against master, but the same change applies

for both the 1.x and 2.x branches.
How SCMs detect binary data:

Git - Look for nul byte in the first 8000 bytes of the buffer.
SVN - Ensure the first 1024 bytes is 15% printable ASCII with no nul bytes

SImple testing:

Add a Bit5-encoded file to a Git repository
git --no-pager diff the repository; this should show that the diff contains binary data
rbt diff the repository; this previously failed, but now succeeds