branchmap-v3: make sure we write the cache after detecting pure-topo mode

We were properly detecting the pure topo-case but in some case the one disk
cache file was never updated with that information.

mercurial/branchmap.py

@@ -1070,6 +1070,7 class BranchCacheV3(_LocalBranchCache):
         for branch, heads in self._entries.items():
             if heads == topo_heads:
                 self._pure_topo_branch = branch
+                self._state = STATE_DIRTY
                 break