Locust GitHub Action should create summary against merged commit

[zomglings]

GitHub's PR UI shows the diff of the base branch with a ref in which the base branch has been merged into the head branch.

Locust summaries take the diff of the base branch with the head branch directly.

When the base branch contains changes not in the head branch, the Locust summary shows different modifications than the GitHub diff.

When the Locust summary is generated for GitHub, Locust should:

1. Try to merge the base branch into the head branch. If this succeeds, set base=base, head=base->head
2. If this merge fails, fall back to base=base, head=head
3. Take diff of base with head.
4. Run Locust summary generator, but replace all links to files with links to diffs (i.e. PR comments).

This last change should probably get its own issue, as it's a substantial improvement in the user experience, and seems like a significant chunk of work.