Release PR into main conflicts when main has commits develop lacks #12

python-multiplatform #10 (release -> main) is CONFLICTING: main and develop have diverged (main has 3 commits develop lacks). The generated release commit has develop as its only parent, so the PR has to merge two histories.

Completion: tools/release/test-sync-release.sh covers a diverged main (release takes main as a second parent, main fast-forwards to release, tree stays develop's) and passes; the release PR is mergeable.