subrepo: fix calling outgoing with multiple paths

The outgoing command is not working with multiple paths.

When recursing into a subrepository all the paths were being passed on to them. However, each subrepository is mapped to only one destination (subrepository state uses the destination to derive the final path). The for loop is responsible for recursing into the subrepository for each destination so we only need to pass the current destination.

This is a follow-up from !828 (closed) to continue discussing it and (hopefully) gather comments from other users interested in this.