Automatically detect default branch of custom repo

[chennes]

Instead of falling back to master, use git's ls-remote command to figure out the default branch.

[github-actions]

Backport failed for main, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin main
git worktree add -d .worktree/backport-428-to-main origin/main
cd .worktree/backport-428-to-main
git switch --create backport-428-to-main
git cherry-pick -x 03b0bb80718fc927344763622e9391494a58401a