Revision 1913310

Move construction of a subject line from the output mechanism over to
the action classes.

Note: this includes a small shim to avoid copy/paste while making
substantial changes. That'll be fixed next.

* tools/hook-scripts/mailer/mailer.py:
  (MailedOutput.start): param is now SUBJECT_LINE
  (MailedOutput.mail_headers): take a full SUBJECT_LINE rather than
    constructing it within this method.
  (SMTPOutput.start, PipeOutput.start): take a SUBJECT_LINE and pass
    that along to the .mail_headers() method.
  (StandardOutput.start): take a full SUBJECT_LINE rather than
    constructing it within this method.
  (Messenger.make_subject): temporary shim to call the code still
    located within OutbaseBase.
  (Commit.generate, PropChange.generate, Lock.generate): construct the
    full SUBJECT_LINE here, rather than within .start()