Baseline PR Tool should close empty PRs

In https://github.com/dotnet/dotnet/pull/7628 (a baseline update PR), the first commit modified the baseline, and the second commit reverted those changes, resulting in a PR with no effective changes. The baseline PR tool should have detected that there were no differences from the target branch, closed the PR, and posted a comment explaining why.

Start with the baseline PR tool and review how it compares a baseline update PR with its target branch, using PR 7628 as the example. Trace the empty-diff case where changes are reverted, then verify that the tool closes the PR and posts a comment explaining that it has no effective changes.

Written by the indexing model from the issue text.