merger to honor series branches
make sure merger respsects the order of series when merging
i.e.
unstable_bionic -> unstable_newerthing -> unstable
This may in fact be tricky to implement, in which case we need to revisit our branch handling (again).
Alternatively: keep concurrent series separate but extend watcher tech to support series branches. also ponder if something besides watcher needs support for concurrent series
Edited by Harald Sitter