Ignore `#hashtag` implicit project tags which have no effect

Summary:
Fixes T5489. Currently, if you make a `#proj` comment on an object already tagged with `#proj`, you get a "no effect" dialog.

Instead, continue if these transactions produce no effect (this is normal/expected, and consistent with `@user`).

Test Plan: Made two `#proj` comments in a row on a revision.

Reviewers: joshuaspence, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5489

Differential Revision: https://secure.phabricator.com/D9745