branchcache: stop storing a repository instance on the cache altogether

We did not really needed it and we do not needs it anymore at all. So lets make
things simpler for consistency and garbage collecting and stop storing it
altogether.

mercurial/branchmap.py

@@ -60,8 +60,8 b' class BranchMapCache:'
     def __getitem__(self, repo):
         self.updatecache(repo)
         bcache = self._per_filter[repo.filtername]
-        assert bcache._repo.filtername == repo.filtername, (
-            bcache._repo.filtername,
+        assert bcache._filtername == repo.filtername, (
+            bcache._filtername,
             repo.filtername,
         )
         return bcache
@@ -80,8 +80,8 b' class BranchMapCache:'
         """
         self.updatecache(repo)
         bcache = self._per_filter[repo.filtername]
-        assert bcache._repo.filtername == repo.filtername, (
-            bcache._repo.filtername,
+        assert bcache._filtername == repo.filtername, (
+            bcache._filtername,
             repo.filtername,
         )
         bcache.write(repo)
@@ -220,7 +220,7 b' class branchcache:'
         """hasnode is a function which can be used to verify whether changelog
         has a given node or not. If it's not provided, we assume that every node
         we have exists in changelog"""
-        self._repo = repo
+        self._filtername = repo.filtername
         self._delayed = False
         if tipnode is None:
             self.tipnode = repo.nullid
@@ -435,8 +435,8 b' class branchcache:'
         )
 
     def write(self, repo):
-        assert self._repo.filtername == repo.filtername, (
-            self._repo.filtername,
+        assert self._filtername == repo.filtername, (
+            self._filtername,
             repo.filtername,
         )
         tr = repo.currenttransaction()
@@ -483,8 +483,8 b' class branchcache:'
         missing heads, and a generator of nodes that are strictly a superset of
         heads missing, this function updates self to be correct.
         """
-        assert self._repo.filtername == repo.filtername, (
-            self._repo.filtername,
+        assert self._filtername == repo.filtername, (
+            self._filtername,
             repo.filtername,
         )
         starttime = util.timer()