simplemerge: add 'mode' kwarg which - when 'union' - suppresses markers This is a step toward adding 'union merge' to the internal merge tool. 'union merge' is a merge strategy which adds both left and right hand side of a conflict region. Git implements this merge strategy which is very practical to have for merging to e.g. the Changelog file. Note: the identifiers with underscores in this commit are all using existing variable names and/or keyword arguments.