Use `diff` command for TFS.
Use `diff` command for TFS.

It turns out that difflib's unified diff implementation, while seemingly okay,
chokes pretty badly with files that don't end in newlines, or start with
byte-order markers. After piling workaround on top of workaround, I've decided
to rip it out and just use the diffutils `diff` command.

Testing done:
Posted some changes containing previously problematic files. Saw that they all
worked.

Reviewed at https://reviews.reviewboard.org/r/7450/

haosdent/rbtools,beol/rbtools,beol/rbtools,davidt/rbtools,reviewboard/rbtools,davidt/rbtools,datjwu/rbtools,reviewboard/rbtools,datjwu/rbtools,reviewboard/rbtools,beol/rbtools,datjwu/rbtools,davidt/rbtools,haosdent/rbtools,haosdent/rbtools