Chrome Version: depot_tools bc9c5ccf00e8e3d3e2937d49ddbc02818f80c351
OS: macOS 10.12.5

What steps will reproduce the problem?
Frequently when making large changes to a CL, I will want to modify the CL description to reflect changes that have been made to the code. Both in Rietveld and Gerrit, this can be done at any time using `git cl description` or the web UI. However in Gerrit, because the description is now part of the patch set, editing it creates another patch set. This clutters the patch set list and messages list.

It would be handy to be able update the CL description on upload, to make the act of "uploading new patch set" and "updating the description" one logical, atomic patch set. Perhaps this could be `git cl upload --edit-description`, which would bring up $EDITOR with the existing description.

What is the expected result?
A way to edit the CL description when uploading a new patch set.

What happens instead?
Have to create another patch set to just edit the CL description, either via web or `git cl description`.

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