Emit single pushed commit as just a single email

When we first deployed git-multimail, the most common complaint was that it generated too much spam.  In particular, lots of folks always pushed a single commit at a time, which made the reference change summary email feel like noise.

One possible improvement here is to make the omission of the reference change email also depend on whether refchange_recipients is equal to revision_recipients.  However, since the revision_recipients could potentially change from revision to revision, that might get complex and/or costly to include.