diff-merges: introduce --diff-merges=on

Introduce the notion of default diff format for merges, and the option
"on" to select it. The default format is "separate" and can't yet
be changed, so effectively "on" is just a synonym for "separate"
for now. Add corresponding test to t4013.

This is in preparation for introducing log.diffMerges configuration
option that will let --diff-merges=on to be configured to any
supported format.

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

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