Fix crash when 'mailbox_in' adds an author
When Commit.email_in()
detects that the incoming mailbox message adds
one or more new authors to a commit, it expands the authors[]
array,
initializing each new element with a virgin Attribution
in which the
date
property is None
. When it later goes to add the new author, it
attempts to compare the incoming date against the date in the virgin
Attribution
and crashes when Date.__ne__
tries accessing None.timestamp
.