Steps to reproduce:

1. Create a commit
2. arc diff
3. Create another commit
4. arc diff

Expected result: the second arc diff creates a dependency from itself to the first arc diff.

Actual result: no dependencies created by default

Fixing this will make submitting large chains of patches for CR a lot more pleasant.

I'm willing to submit a patch but I first want to determine what default behavior people want. I think it would be reasonable to just automatically append "Depends on" to the commit message generated by arc diff.