Show full names for attached diffs and tasks in email in Maniphest

Summary:
Ref T2217. Use `getLinkName()` instead of `getName()` so that we get, e.g.,

  alincoln attached a revision: D123 Chop some logs

...instead of:

  alincoln attached a revision: D123

Test Plan: Attached stuff, looked at the email, saw full object name.

Reviewers: btrahan, asherkin

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2217

Differential Revision: https://secure.phabricator.com/D7127