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.