convert: use 'default' for specifying branch name in branchmap (issue4753) A fix for issue2653 with 1d155582a8ea introduced a discrepancy how default branch should be denoted when converting with branchmap from different SCM. E.g. for Git and Mercurial you need to use 'default' whilst for Perforce and SVN you had to use 'None'. This changeset unifies 'default' for such purposes whilst falling back to 'None' when no 'default' mapping specified.