Compare against PR head commit instead of merge ref

[microdev1]

When a commit with checks is found, compare it with head commit instead of merge ref.

[dhalbert]

I'm assuming this solves some problem, but could you give an example of what it fixes?

[microdev1]

The goal is to diff between a PR's latest commit (head commit) and the last commit with a workflow run when available.

The PR's merge ref was being used instead of the PR's latest commit which caused inclusion of changed files from the PR's base branch. The merge ref contains changes in the PR plus the base branch which might have been updated since that last commit.

Suppose two PRs A and B with base branch being main.
PR-B is merged into main prior to PR-A.
Now comparing PR-A's merge ref with a commit in PR-A will include changes that were in PR-B.
These additional changes are undesirable in the current context.

[dhalbert]

Thanks for the explanation!