feature request: predict squash-merge commit on target branch and lint it as well

[SvenStaehs]

This action checks the commits included in a PR before the merge, and it checks the push to target branch after the merge, but it does not warn the user if their PR would cause commitlint violations when merged to the target.

This happened recently in my project: we allow only squash-merge of PRs, with "default to pull request title and commit details" as commit message, so the commit on target branch has nothing to do with the commits we check during PR validation. Of course there is nothing preventing the user from changing this default and entering whatever they wish, but commitlint-github-action could predict the default commit message on the target branch and run it through commitlint.

There are multiple ways the merge message on target branch can be generated:

• In my scenario, the commit header would be "${{ github.event.pull_request.title }} (#${{ github.event.pull_request.number }})" and commit body would be a list of all commits (with their headers prefixed with "* ")
• other options for default commit message include the body of the commit being the PR description instead of a list of commits, and a "default message" whatever github decides that is.
• if rebase is allowed, there won't be a separate message at all (I believe)
• there may be other defaults to check, e.g. Dependabot ignores the default and forces the PR description into the commit body (with quite long lines due to it including links...)

So I guess I mainly want to start a discussion here if these things should be handled by this action at all or not. I know about amann's action to check the title, but it does not actually perform a commitlint, especially it does not check the length at all (hence our recent issue that we ended up with a merge commit on main branch that commitlint complained about... At least it was still parsable by semantic-release, so it didn't break anything but the commitlint step.)

Alternatively, it would be nice if we could just provide arbitrary text to be linted. I tried generating an empty commit with the predicted message header, but during PR-triggered workflows the action reads the list of commits in the PR with octokit, so I have no way of adding my mock commit to that list. If I could provide a "text-to-lint", that would solve my issue. Or a "commit-hashes" list that is used instead of reading it from PR, maybe this is useful for other circumstances as well?

Other workarounds include:

• running commitlint directly just to check the title
• check the title with amann's action and check the length with a separate bash step but I would far prefer using the same action that will double-check the push to target branch so I know the configuration is identical (e.g. for the length check with bash I won't be trying to read the commitlint.rc and will just hardcode the value ;)).