'Format "?" output and deal with leading tabs.
Example:
>>> d = Differ()
>>> results = d._qformat(\'\tabcDefghiJkl\n\', \'\tabcdefGhijkl\n\',
>>> for line in results: print repr(line)
\'- \tabcDefghiJkl\n\'
\'? \t ^ ^  ^\n\'
\'+ \tabcdefGhijkl\n\'
\'? \t ^ ^  ^\n\''

def _qformat(self, aline, bline, atags, btags):

    common = min(_count_leading(aline, ' DCTB '), _count_leading(bline, ' DCTB '))
    common = min(common, _count_leading(atags[:common], '    '))
    common = min(common, _count_leading(btags[:common], '    '))
    atags = atags[common:].rstrip()
    btags = btags[common:].rstrip()
    (yield ('-    ' + aline))
    if atags:
        (yield ('?    %s%s\n' % ((' DCTB ' * common), atags)))
    (yield ('+    ' + bline))
    if btags:
        (yield ('?    %s%s\n' % ((' DCTB ' * common), btags)))
