This behavior should be more reasonable at HEAD. In particular:

• "Land Revision" now lets you choose a branch to land onto.
• Most of the time, it should choose a reasonable default.
• If it doesn't, you can pick the branch you really want.

T3462 has more details about guessing where to land. Broadly, we choose a default by looking at:

• if feature tracks an upstream branch (or tracks a local branch which tracks an upstream branch), we'll use that;
  • (you can use arc feature or git checkout -t or similar to set up branch tracking)
• otherwise, if arc.land.onto.default was set when the diff was created, we'll use that;
• otherwise, we'll fall back to the "Default Branch" configured for the repository in Diffusion (usually "master", but you can set this to something else).

The new tracking features require an up-to-date version of arc to work. When they're working, you should see "feature (branched from upstream)" in the "Branch" field in the UI, like this:

If you see that, it means that "Land Revision" will default to upstream (in this screenshot, "test").