Improve Herald commit emails

Summary:

• Show the canonical (i.e., shorter) commit identifier in the subject.
• For commits without a revision, put the commit summary in the subject.

Test Plan: Ran "scripts/repository/reparse.php <commit> --herald" for a number of different commits (with revision, without revision); got more useful email subjects.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T1028

Differential Revision: https://secure.phabricator.com/D2004