diff-merges: add old mnemonic counterparts to --diff-merges

diff-merges: add old mnemonic counterparts to --diff-merges

This adds --diff-merges={m|c|cc} values that match mnemonics of old
options, for those who are used to them.

Note that, say, --diff-meres=cc behaves differently than --cc, as the
latter implies -p and therefore enables diffs for all the commits,
while the former enables output of diffs for merge commits only.

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