Fix Git clone errors when SSH host key added

If you're cloning a repository over SSH, and it's the first time SSH
has connected to that host, a warning is emitted to stdout saying
something along the lines of:

    Warning: Permanently added the RSA host key for...

If cloning a repository fails for any reason, Babushka has a feature
where it tries to "pretty up" the error message, and in the process it
strips everything after the first line of the stderr output from Git.
If this warning about the SSH host key is printed, then cloning the
repository fails for any reason, all the user will see is the warning,
and no details of the actual error at all. This can be very confusing.

This change removes the warning from the output before doing the whole
"only show the first line" thing, so the first line is what it should
normally be (when the warning isn't present).