Use --no-color with log invocations

Fixes a silent failure to show anything when invoked as 'git -p tbdiff
...', which happened because the underlying git-log would decide to
use colors, breaking our pattern matches.
Use /usr/bin/python2

Johan Herland already pointed out in May that modern systems have
/usr/bin/python[23] symlinks for specific versions, and /usr/bin/python
at the user's discretion.

Let's actually go with the times and do it that way.
Convert the readme to markdown

I have no idea if this actually suffices.
Turn the README into more of a manpage

Instead of the a bit informal "how did we get here" description, start
out with more manpage-like instructions.