subrepo: propagate non-default path on outgoing

I caught this problem when setting a second repository manager for code review and including all repositories (parents and sub-repositories). The parent repo was being compared to the right remote, however the sub-repositories where being compared with the default repository.

Let me know if there is a better place to do this fix, I tried to do something as close as in 5dbff89cf107, but I noticed that the logic for outgoing is different compared to the logic for push when sub-repositories are involved.

Another thing that I noticed is that outgoing does not work for multiple paths when there are sub-repositories, so I ignored this case in this commit. Pushed a new commit to fix this.