Add branchtags function with cache
- cache stores tipmost node and rev and node,label pairs
- if cache is out of date, scan new changesets
- write new cache out after scan
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)