Spent all day fixing sync in adjusted branches. I was lost in the weeds for a long time. Eventually, drawing this diagram helped me find my way to a solution:

origin/master    adjusted/master     master
A                                    A
|--------------->A'                  |
|                |                   |
|                C'- - - - - - - - > C
B                                    |
|                                    |
|--------------->M'<-----------------|

After implementing that, syncing in adjusted branches seems to work much better now. And I've finally merged support for them into master.

There's still several bugs and race conditions and upgrade things to sort out around adjusted branches. Proably another week's work all told.