maps: add the "branchmaxrevnum" method to RevMap Iterating the RevMap can be very expensive for large repos. To prevent future code from iterating the possibly huge RevMap causing performance issues, it's a good idea to clean all iterating code outside RevMap. This patch adds a method used by "get_parent_svn_branch_and_rev". In the future, we will have a SqliteRevMap, which will have an efficient implementation for this query, and it will disable "__iter__" to prevent future performance issues.