diff-merges: introduce log.diffMerges config variable

diff-merges: introduce log.diffMerges config variable

New log.diffMerges configuration variable sets the format that
--diff-merges=on will be using. The default is "separate".

t4013: add the following tests for log.diffMerges config:

* Test that wrong values are denied.

* Test that the value of log.diffMerges properly affects both
--diff-merges=on and -m.

t9902: fix completion tests for log.d* to match log.diffMerges.

Added documentation for log.diffMerges.

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