There are two issues:
1)
```
[10046 eax@EagleMac ~ !1!]%arc diff --base foo
Usage Exception: `arc diff` is only supported under git, hg, svn.
```
but arc help diff says
```
--base rules
Supports: git, hg
Additional rules for determining base revision.
```
is --base supported for svn or not?
2) `Additional rules for determining base revision.` provides no context for what this might be and I can find nothing under [[https://secure.phabricator.com/book/phabricator/article/arcanist_diff/]]