Situation: You have a local branch (uploaded to gerrit) which fails on the try bots because of an unrelated bug you accidentally triggered. Another developer fixes the bug upstream, that CL has not landed yet. You want to rebase your CL on that other CL to test that it indeed fixes the error you triggered.

Steps we did:

1) Checkout the other CL, using the command provided as "Checkout" in "Download" in Gerrit: git fetch https://chromium.googlesource.com/v8/v8 refs/changes/21/964221/1 && git checkout FETCH_HEAD

2) Create a branch: git checkout -b upstream-fix

3) Connect it to the uploaded CL: git cl issue 964221

4) Checkout your local branch which triggered failures: git checkout my-change

5) Set upstream: git cl upstream upstream-fix

6) Rebase: git rebase -i @{upstream}

7) Verify that everything looks correct: git map-branches -vvv
--> both branches are connected to the right gerrit CLs; "upstream-fix" has exactly the same sha1 as gerrit is showing; "my-change" depends on it. The diff from "my-change" to "upstream-fix" is the same as the one uploaded to gerrit.

8) Upload: git cl upload -d -t "depend on upstream fix"

This produces the error message:
============================================================================================
[...]
Error after CL description prompt -- saving description to <home>/.git_cl_description_backup


Upload upstream branch upstream-fix 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.)
============================================================================================

When I saw this error message before, it was indeed because the dependant branch diverged from the uploaded version. But in this case, it's exactly the same sha1 hash.

Expected behaviour: upload should work, with the same diff as before, but an additional dependence in gerrit.