This would boil down to merge level messages. Not all projects squash commits, so what you actually care about here are the mr/pr level messages, which might be approximated by the commit message on the merge commit, but actually probably will just say merge {branchname}. When you bisect with parent=0 you only see merge commits or ff directly against main branch.
Putting what im saying another way, in a project with pull requests, commit messages are redundant with the text typed into the pr and the comments on same. We should just carbon copy those onto the merge commit and forget per commit messages.
Putting what im saying another way, in a project with pull requests, commit messages are redundant with the text typed into the pr and the comments on same. We should just carbon copy those onto the merge commit and forget per commit messages.