--diff option should be smarter about reporting duplicate errors

[glyph]

Presently, if you have a file that contains lots of errors, and you fix a few errors at the top, twistedchecker --diff will report that you have introduced all the errors after your fixes, if you have lengthened or shortened the code at all. #41 suggests a specific solution to this, but I wanted to open an issue to capture this specifically.

The Twisted buildbots have a somewhat smarter algorithm, which reports fewer spurious / duplicate errors. It is somewhat silly that we have two ways of implementing this ratchet; in order to facilitate moving the syntax checks to Travis-CI or another hosted service, I think that the logic implementing the always-fewer-errors ratchet should move into twistedchecker rather than buildbot.

This PR in vertex twisted/vertex#38 points towards a direction for quickly determining a baseline given a git repo on Travis.

[adiroiban]

There is this project https://pypi.python.org/pypi/diff_cover

maybe we should give it a try and see if we can use it with twistedchecker