git-imerge, git-when-merged, git-delete-merged-branches: fix test on Monterey

[cho-m]

• Have you followed the guidelines for contributing?
• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install --build-from-source <formula>)? If this is a new formula, does it pass brew audit --new <formula>?

---

Fix some Monterey test failures in #109777 due to git init using main branch.

[BrewTestBot]

🤖 A scheduled task has triggered a merge.