[IanCal]

Would you not just skip those? Skipping non-compiling code is the canonical example of "git bisect skip".

You could also have a little script that does something like:

    # Usage gbisect_prs bad good
    git bisect start
    for commit in "git log --since good --until bad"
        if not commit.message.startswith("Merge pull request #"):
            git bisect skip commit.hash

Turned into real code, obviously, but it'd tell git to ignore any commit that's not a PR merge.

You could also just include this into your bisect script if you're not doing it by hand, return 125 if it's not a commit you want to test.