diff-merges: revise revs->diff flag handling

diff-merges: revise revs->diff flag handling

Do not set revs->diff when we encounter an option that needs it, as
it'd be impossible to undo later. Besides, some other options than
what we handle here set this flag as well, and we'd interfere with
them trying to clear this flag later.

Rather set revs->diff, if finally needed, in diff_merges_setup_revs().

As an additional bonus, this also makes our code shorter.

Signed-off-by: Sergey Organov <redacted>
Signed-off-by: Junio C Hamano <redacted>