revision: provide implementation for diff merges tweaks

revision: provide implementation for diff merges tweaks

Use these implementations from show_setup_revisions_tweak() and
log_setup_revisions_tweak() in builtin/log.c.

This completes moving of management of diff merges parameters to a
single place, where we can finally observe them simultaneously.

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