branchmap: display filtername when `updatebranch` fails to do its jobs

We have a very handy assert at the ends of `branchmap.updatecache` that check
the resulting branchmap is actually valid.

I know we do not like assert in mercurial but this one is very handy for
debugging. There is really not reason for `branchmap.updatecache` to have this
kind of issue but this happened and handful of time during the development of
this or introduction of other related feature. I advice to keep it around until
we are a bit more confident with the new code.

mercurial/branchmap.py

@@ -79,7 +79,7 b' def updatecache(repo):'
     if revs:
         partial.update(repo, revs)
         partial.write(repo)
-    assert partial.validfor(repo)
+    assert partial.validfor(repo), filtername
     repo._branchcaches[repo.filtername] = partial
 
 class branchcache(dict):