Usually, I can do the following:

1. git cl upload a branch, call it oldbranch
2. git checkout -t -b newbranch
3. git cl upload newbranch
4. Gerrit will show a dependency on the CL uploaded from oldbranch

Lately, however, this doesn't seem to be working. I get an error, even though git cl status shows that the upstream branch does have an associated review URL:

Upload upstream branch [branchname] first.
It is likely that this branch has been rebased since its last upload, so you just need to upload it again.
(If you uploaded it with --no-squash, then branch dependencies are not supported, and you should reupload with --squash.)