Check new code only

[robbat2]

This is going to take a bit of doing, but was wondering about the conceptual parts of it first.

When adding pre-commit to an existing codebase, there can be a lot of errors, and you still want to prevent introduction of new errors, plus whittle down the existing errors over time.

To that end, a mode that excludes errors due to pre-existing code, and only flags errors in new commits.

I can see a few ways that this could be mostly accomplished, but not reliably:

• parse/filter checker output for lines that were added (EASY)
• run checkers twice on old & new code, adjusting line numbers from old to new, and exclude errors that occured in both (MEDIUM)
• teaching checkers to only care about a range of lines (HARD)