collect_task_ids(): remove #, trigger only on Tnnnn
Context: @jbond brought this up in IRC:
<jbond> hi all not sure if this has allready been reported but seems
like CodeReviewBot is confusing gitlab issues with phab tasks
<jbond> https://phabricator.wikimedia.org/T1#8953263
While issues shouldn't generally be enabled, this is still a bug. Based on discussion elsewhere, I think we decided that "Bug: #nnn" is generally the old format for Bugzilla bugs and probably shouldn't be repurposed for linking Phab, but also I guess if new stuff uses it on GitLab, we don't want to confuse it for a Phabricator task id.