We mostly deleted some files, so we can just ignore changes to these. The annoying part will be merging Meson files.
@kennylevinsen has tracked our snapshot back to https://github.com/systemd/systemd/commit/c4e48030cf299b56668b1cd360c4ec93e7f97b20, at least for the sd-bus part.
We'll probably want to manually port patches from the systemd tree to basu. Referencing the original commit in each ported commit would be ideal. We probably care more about bugfixes than enhancements.