Use merge base when calculating changed files

[KyleFromNVIDIA]

Description

get-pr-info.outputs.base.sha does not actually give the merge base, but merely the tip of the target branch. Calculate the merge base and pass it to the changed-files step.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[KyleFromNVIDIA]

/merge

[vyasr]

Seems sensible enough. Presumably the error before was occurring because when the PR branch was outdated the base SHA pulled from the target branch was not actually in the history of the current branch?

[KyleFromNVIDIA]

Presumably the error before was occurring because when the PR branch was outdated the base SHA pulled from the target branch was not actually in the history of the current branch?

Yes, and in addition, even if it had been present, it wouldn't have correctly calculated the changed files anyway. The tj-actions/changed-files action doesn't compute the merge base on its own for push events. It does for pr events, but that doesn't help us because we're using our "copy the PR to a new branch" pattern. So the only choice is to calculate the merge base ourselves and pass it to tj-actions/changed-files.