In continations (that use spaces for alignment), don't allow the number
of leading tabs to change. Previously only removal of tabs was
disallowed, but addition doesn't make sense either, as only spaces
should be used for further alignemnt.

This catches situations like this:

  |<-tab->all_work_and_no_play (makes,
  |<-tab->                      jack,
  |<-tab-><-tab->               a dull boy);