Repro steps:
Set up repo with
master -> A -> B
Where A and B are dependent branches. A doesn't have a revision associated (say we wanted to skip review) and B has a revision associated.
```
git checkout master
git checkout -b A
# do edit A
git commit -a -m "Commit A"
git checkout -b B
# do edit B
git commit -a -m "Commit B"
arc diff HEAD^
```
Now, if you go to branch A and try to land, it will block on the revision from commit B, which is odd behavior in my opinion. When in branch A, arc has no business mentioning commits that only exist on branch B
```
git checkout A
arc land
```
I think two potential valid behaviors might be these (obviously open to other options).
1) Land branch A successfully
2) Fail to land because there's no revision in the tree for A
I believe this is the code that causes the issue.
https://github.com/phacility/arcanist/blob/master/src/repository/api/ArcanistGitAPI.php#L1038