The branch picker collapses remote branches that are tracked by a local
branch (e.g. `origin/main` when local `main` tracks it). That makes
sense for checkout, where the remote ref is redundant, but it was also
applied to the branch diff base picker, where `main` and `origin/main`
can point to different commits. As a result, once you switched the diff
base from `origin/main` to `main`, there was no way to switch back.

The tracked-remote collapsing is now only applied in checkout mode (no
changed here). The select (diff base) picker lists both local branches
and their remote upstreams.

Release Notes:

- Fixed remote branches (e.g. `origin/main`) not appearing in the branch
diff base picker, making it impossible to switch back after changing the
base to another branch.