Caching for baseline linting

[akaihola]

Is your feature request related to a problem? Please describe.
Linting became slower in #393 due to running each linter twice.

Describe the solution you'd like
In local development, the developer often re-runs Darker multiple times for the same branch, having effectively the same rev1 revision each time. The linting output is identical every time since baseline linting is done in a fresh clone of the repository at rev1. Linting output could be cached so the time spent on running linters could drop to half in the extreme case.

Describe alternatives you've considered
We could decide to not cache, and accept the slowdown.

[akaihola]

Moved to akaihola/graylint#27.