There are some redundancies in the validation steps, and that is to
preserve current error messages behavior
Also few specs have to be changed in order to fix madness in validation
logic.
The repository push email can be very expensive to generate, especially
with syntax-highlighted diffs. Instead of generating the email for each
recipient, generate one email object and reset the Message-Id and To
headers for each recipient. (Cloning would also be expensive in the case
of large emails, although probably not as bad as generating from
scratch.)
