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.
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