Set up a test repo with this structure:
```
| * Commit B1, on branch "subfeature"
| /
| * Commit A1, on branch "feature"
|/
* Commit M1, on branch "master"
|
```
In `subfeature`, I tried:
* `arc which --base 'git:branch-unique(master)'`
* `arc feature`
After that, I detached my HEAD (don't worry, I got better) and tried again.
Nothing looked broken.
(Tested with git 1.7.2.5 and 2.5.0.)