diff-merges: split 'ignore_merges' field

diff-merges: split 'ignore_merges' field

'ignore_merges' was 3-way field that served two distinct purposes that
we now assign to 2 new independent flags: 'separate_merges', and
'explicit_diff_merges'.

'separate_merges' tells that we need to output diff format containing
separate diff for every parent (as opposed to 'combine_merges').

'explicit_diff_merges' tells that at least one of diff-merges options
has been explicitly specified on the command line, so no defaults
should apply.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>