git allows you to set which paths to use for the git server programs
when connecting over ssh; and we want to provide something similar.

We do this by providing a factory function which can be set as the
remote's transport callback which will set the given paths upon
creation.

Our ssh tests assume that the server supports password authentication
in a few places. This is convenient as we're not testing authentication
methods, but what happens around them.

Tell sshd on OSX to accept this form of authentication.

Instead of completely giving up on the first failure, ask for
credentials as long as we fail to authenticate.

An empty string is not a valid number, and some shells complain.

Check instead if $COVERITY is non-empty, which is a common convention
and what we're doing anyway.