branchmap: don't add branch entries if there are no heads...
branchmap: don't add branch entries if there are no heads
We definitely don't want any empty entries to be present in repo.branchmap()
just for the sake of not breaking test-notify.t.
No test changes required because the previous patch made notify extension to
not raise any tracebacks in case of RepoLookupErrors.
Differential Revision:
https://phab.mercurial-scm.org/D12135