Prevent Differential changeset HTML anchors from colliding with comment anchors

Summary:
Fixes T12970. This is easier than I expected, and appears to occur in only one place.

This prevents a change from ever generating with an anchor like #12345678, which is ambiguous because it may be a comment anchor.

Test Plan: Viewed a revision, saw new change-xxxyyyzzz anchors, clicked one, got jumped to the right place.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12970

Differential Revision: https://secure.phabricator.com/D18465