Changes to test merge commit generation for pull requests

To reduce delays when determining the mergeability for a pull request and improve system reliability, we’ve changed the frequency at which we generate test merge commits for open pull requests.

Test merge commits will only be generated when one of the following is true:

• Changes are pushed to the pull request branch.
• The merge base between the pull request and base branches changes.
• The current test merge commit is older than 12 hours.

Previously, test merge commits were also generated when viewing the pull request page.

This change does not impact mergeability checks, reporting of merge conflicts, or rule enforcement.