Allow selective caching for --fix and --diff

[charliermarsh]

Summary

If a file has no diagnostics, then we can read and write that information from and to the cache, even if the fix mode is --fix or --diff. (Typically, we can't read or write such results from or to the cache, because --fix and --diff have side effects that take place during diagnostic analysis (writing to disk or outputting the diff).) This greatly improves performance when running --fix on a codebase in the common case (few diagnostics).

Closes #8311.
Closes #8315.

[github-actions]

PR Check Results

Ecosystem

✅ ecosystem check detected no linter changes.

[zanieb]

Nice. Should we retain a note about why we don't always cache?

[lukaspiatkowski]

I can confirm this solves #8311, second run of ruff --fix took 0.13s on my repo as expected.

[charliermarsh]

Nice, thanks for testing on main! Appreciate it!

[charliermarsh]

I'd expect ruff --fix to still be much slower if most files have diagnostics, since we only skip the cache for files with no errors, but in practice that's very rare in real-world projects.