From @bitglue:
```
pfrost@zoom:~/Documents$ git clone git@git:puppet-macpros_jenkins test
Cloning into 'test'...
warning: You appear to have cloned an empty repository.
Checking connectivity... done
pfrost@zoom:~/Documents$ cd test/
pfrost@zoom:~/Documents/test$ arc patch D7
Exception
Command failed with error #128!
COMMAND
git diff --no-ext-diff --no-textconv --raw 'HEAD' --
STDOUT
(empty)
STDERR
fatal: bad revision 'HEAD'
(Run with --trace for a full exception trace.)
pfrost@zoom:~/Documents/test$
```
We probably need some special casing for empty repositories (like we have in various other places), since HEAD isn't meaningful when a repo is empty.