Agents window: Sync Changes shows up for a branch tracking a local branch, and fails when clicked

1. In a repo open in an Agents window session, create a branch that tracks a local branch instead of a remote one:
```
git checkout -b x
git branch --set-upstream-to=main
```
2. Commit something on `x` and something else on `main` so they diverge.
3. The title bar now shows "Sync Changes 1↓ 1↑". Click it.

**Expected:** Sync Changes isn't offered, since there's no remote to sync with.

**Actual:** The button is there, and every click fails with "Could not resolve the remote for the branch".

Version: 1.137.0
Commit: 645f29cc3176500b4b5762ba887cf2a7f0ffdf2c
OS: Linux x64 (Pop!_OS 24.04)