Extract a method ReferenceChange.get_subject().

Generate the subject from a separate REFCHANGE_SUBJECT_TEMPLATE rather
than directly from REFCHANGE_HEADER_TEMPLATE.  This is a first step to
generating better subject lines that depend on the change type.
Use separate subject templates for each change type.

This brings us a step closer to making the subject messages
internationalizable, by avoiding the assumption that the variation
between change types can be captured by expanding "%(change_type)sd"
into the past participle of the change type.

Leave change_type in the expansion values anyway, in case somebody
else is already using it.
Mention SHA1s in refchange subjects.

Aside from being convenient, this might help prevent some email
clients (e.g., gmail) from trying to group all updates to a branch
into a single thread based only on the fact that they have the same
subject.

Suggested by Matthieu Moy <Matthieu.Moy@imag.fr>
Merge branch 'sha1s-in-subject'

Include short SHA1s in the email subject lines to discourage email
clients from threading unrelated messages together.