make MTN_MERGE_DIFFUTILS a variable which can contain a
comma-separated list of options and currently implement 'partial'
and 'showall'. Also improve portability for 'partial' by using
execute_redirected() instead of an 'sh' based hack
diff3 return 0 on no conflicts (should not happen in this context)
and 1 on conflicts (which is what we expect here) and 2 on error, so
fail on 2 only; additionally require diff(1) in PATH for sdiff(1)
call