I ran into an issue where an `arc diff` had my recent commits but not the full history. To reproduce this issue, do the following:
```
git checkout -b branch1
touch foo
git add .
git commit -m 'foo'
git checkout -b branch2
git checkout branch1
touch bar
git add .
git commit -m 'bar'
arc diff --preview
```
Expected result: the arc diff includes both 'foo' and 'bar'.
Actual result: the arc diff only includes bar, not foo.
It looks like, when branch2 is created (as a sub-branch of branch1), arc diff thinks that it only needs to diff the HEAD against the commit when branch2 was created rather than against master.