Include SHA of test merge commit in PR generator for GitHub

[purajit]

Component: Pull Request Generator

Request: Currently, when using the PR Generator with GitHub, we are provided with the head_sha. Additionally being provided with merge_commit_sha would be extremely useful.

Motivation: The default behavior of a lot of widely-used GHAs, such as actions/checkout and docker/metadata-action, is to use the test merge commit sha rather than the head sha. Providing this value would mean users have a way of working with PR generator without having to override this behavior throughout their stack, and seeing predicatable values being propagated everywhere.

I'm unfamiliar with GitLab and Gitea but I think based on some quick searching that they too create test merge commits, but at least for GH, it seems like it would require a change

• In the client to extract merge_commit_sha
• To param population
  

Other links:
MergeCommitSHA in the GitHub client
Argo docs for template and params
Information about test merge commit SHA and how it changes