Fixes T9543. Fixes T9658. Ref T3855.
Major functional change is that you can have a sequence of branches like:
origin/master -> notmaster -> feature1
...where they track each other, but you named your local master something else. Currently, we resolve only one level of upstreams, so we try to land onto "notmaster" in this case, which is wrong.
Instead, keep resolving upstreams until we either hit a cycle, don't have another upstream to look at, or find someting in a remote. In this case we'll eventually find "origin/master" and select "origin" as the remote and "master" as the target.
Other minor changes:
- Make this selection process explicit.
- Make the help 3000x longer.
Also fix a bug where we could incorrectly try to tell Differential to update awith --preview.