dev/ci: make changedFiles calculation smarter in PRs

Created by: bobheadxi

When preparing changedFiles we perform a diff against main. Right now, if you push the following commits to a PR separately:

1. A bunch of Go changes
2. A bunch of web app changes
3. A bunch of docs changes

In commit 1, Go tests run, then in commit 2 Go and web tests run (well, actually all tests run but assuming we improve that too) and in commit 3 everything is in the pipeline. Maybe we could find a way to perform the diff against previous runs of the same branch?

Related: https://github.com/sourcegraph/sourcegraph/pull/25229