R1:e81637a6c686

Fix some issues with the "Explain Why" dialog

Summary:
Ref T11051. This is still not as clear as it should be, but is at least working as intended now.

I believe this part of the code just never worked. The test plan on D10489 didn't specifically cover it.

Test Plan:
Did this sort of thing in a repository:

```
$ git checkout -b featurex
$ echo x >> y
$ git commit -am wip
$ arc diff
```

Then I simulated just pushing it (this flow is a little more involved than necessary):

```
$ arc land --hold
$ git…