With opportunistic ref updates, git has introduced the concept of having
base refspecs *and* refspecs that are active for a particular fetch.
Let's start by letting the user override the refspecs for download.
The online::push::notes test pushes a note but leaves it hanging
around for other tests to stumble across when they're validating
that they're seeing the refs they expect to see. Clean it up on
exit.
For urls where we do not specify a username, we must handle the case
where the ssh transport asks us for the username.
Test also that switching username fails.
