R1:d57c1d6ce2f5

Make hashtags at the end of bolded text render properly

Summary: Fixes T10096. The `**` was being parsed as part of the hashtag, so `**#asdf**` interpreted `#asdf**` as a hashtag.

Test Plan: Unit test; bolded stuff with hashy contents.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10096

Differential Revision: https://secure.phabricator.com/D15006