Currently, if one edits one's local CL description (e.g., `git commit
--amend`), there is no path for that update to get pushed to Rietveld.
This adds a "+" sentinel to the "-n" flag for `git cl description` that
tells it to load description content from the local commit.

The description should match what would be generated by "git cl upload".

BUG=None
TEST=local

Review-Url: https://codereview.chromium.org/2307463002