diff-merges: move specific diff-index "-m" handling to diff-index

diff-merges: move specific diff-index "-m" handling to diff-index

Move specific handling of "-m" for diff-index to diff-index.c, so
diff-merges is left to handle only diff for merges options.

Being a better design by itself, this is especially essential in
preparation for letting -m imply -p, as "diff-index -m" obviously
should not imply -p, as it's entirely unrelated.

To handle this, in addition to moving specific diff-index "-m" code
out of diff-merges, we introduce new

  diff_merges_suppress_options_parsing()

and call it before generic options processing in cmd_diff_index().

This new diff_merges_suppress_options_parsing() could then be reused
and called before invocations of setup_revisions() for other commands
that don't need --diff-merges options, but that's outside of the scope
of these patch series.

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