[anomiex]

Changes proposed in this Pull Request:

When processing a tag push and PATHS is set, we need to find the merge
base to get the diff to determine if the paths are touched.

But it's a lot faster to fetch 100 revisions at once than to fetch 10
revisions 10 times. So instead of fetching to each PR's merge base,
which will likely enough fetch a few revisions for every PR beyond a
certain age, fetch arbitrary blocks of revisions.

Jetpack product discussion

None

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• You could set the relevant environment variables and run the script manually. Or merge then watch the run on the next tag push.

[github-actions]

Thank you for your PR!

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