urlutil: add a new get_unique_push_path
This function is dedicated to call that needs a single destination. Currently
most caller actually need that since few actually support multiple destinations
(the most importants hg push and hg outgoing do). So having a clear API
point for that will be important when the time comes to have a single [paths]
alias resolving to multiple urls.
Differential Revision: https://phab.mercurial-scm.org/D10407