What steps will reproduce the problem?
1. `git cl upload` a CL with C++ namespace separator in the subject e.g. A::B

What is the expected output?
A is not a hashtag.

What do you see instead?
A and everything before it gets set as a hashtag.

Example: https://chromium-review.googlesource.com/c/chromium/src/+/1280062

This is unlikely a Gerrit issue due to https://bugs.chromium.org/p/gerrit/issues/detail?id=9874.