templatefilters: wrap all filters in dedicated functions
This will highly simplify the docstring integration. I measured "hg log
--style=changelog" duration on mercurial itself and could not detect any
difference.
http://example.com/no/anchor, branches: (None, [])
http://example.com/an/anchor, branches: ('foo', [])
http://example.com/no/anchor/branches, branches: (None, ['foo'])
http://example.com/an/anchor/branches, branches: ('bar', ['foo'])
http://example.com/an/anchor/branches-None, branches: ('foo', [])