ci(darker): improve comparison refs for focused errors

[p2edwards]

👀 Preview steps

1. 🔴 run darker on a branch with an old base ref.
   • darker fails (from changes in main) ❌
2. 🟢 after this fix
   • darker passes (no .py files changed on that pr branch) ✅
3. 🟢 edit tasks.py to break a flake8 rule
   • darker fails, but only for tasks.py, not other ✔️

Screenshots from test commits in #5295

1. 🔴🖼️ unrelated files linted

   
2. 🟢🖼️ no unrelated files linted

   
3. 🟢🖼️ related file(s) linted

   

💭 Notes

Change which commits we compare in darker.yml so that their diffs don't include changes introduced by other branches.

More detailed explanation here

xref internal chat

[rgraber]

Repro steps:

1. cut a branch off main (fake-main)
2. cut a branch off fake-main (fake-innocent)
3. cut another branch off fake-main (fake-problem)
4. Commit code to fake-innocent that won't cause a linting error (eg a short comment)
5. Push the code and open a PR for fake-innocent against fake-main
6. Commit code to fake-problem that will cause a linting error in a different file than the one changed in fake-innocent. A comment over 88 characters will work
7. Push the code and open a PR for fake-problem against fake-main
8. The CI for fake-problem will fail. Merge it to fake-main anyway
9. Make another change in fake-innocent (do not touch the file changed in fake-problem)
10. Commit and push the change
11. 🔴 darker will fail on the PR for fake-innocent based on the problem introduced by fake-problem, even though fake-innocent did not modify those files
12. check out this branch
13. repeat steps 1-10, starting with this branch instead of main
14. 🟢 darker should pass on the "innocent" branch

[rgraber]

Note: this behavior was slightly different than expected. The old darker action looked at the latest commit to main when the PR was opened, regardless of when the branch was cut. When testing this fix, we expected it to look at the latest commit before the branch was cut. This means darker would only fail if a PR for the "innocent" branch was opened before the "problem" branch was merged.

Thus, to reproduce the bug, make sure you open the "innocent" PR first, then merge the "problem" PR, then re-run CI for the "innocent" PR.

[rgraber]

2 suggestions

[rgraber]

LGTM