[code-quality] Making git and pylint interact

[Skip Montanaro]

> I'm not entirely satisfied yet,
> and if anybody has anything further to add, I'm still interested.

My thinking is that if the code differences don't tell you what
changes in pylint output are important, then maybe either

* pylint results should be stored under version control so you can diff them

or

* pylint should be run on both the reference checkin and the current
state of your code

In either case, you'd need to do a bit of a tricky dance when
messages. To avoid spurious diffs, you might have to substitute "NNN"
for your line numbers, compare, then restore the actual line numbers
in the messages you present to the user.

Skip