I've twice had this problem.
repo start nxxxxxx xxxx-firmware
<edit>
git add
git commit
git cl upload
I then am presented with the error:
jglinux1:xxxx-firmware(nxxxxxx) jglasgow$ git cl upload
Unable to determine default branch to diff against.
Either pass complete "git diff"-style arguments, like
git cl upload origin/master
or verify this branch is set up to track another (via the --track argument to
"git checkout -b ...").
jglinux1:~ jglasgow$
If I try
git cl upload cros/master
then I get a little further. It fires up an editor an allows me to edit the CL description, but after editing the description it dies again with the same message.
-Jason