bin/pick: track patches that are backported and applied

Currently, if a patch is backported and marked as backported, or if it is manually applied, then the pick-ui will not associate the commit in the main branch with the commit in the stable branch. This means that any commit that fixes: that commit will not be considered.

This series (with a bunch of reworks) attempts to rectify that by taking advantage of an attribute that was added when the script was created but never properly wired up, and then teaching the script to both populate that attribute, and consider it when doing backports.