v2writable: avoid mm_tmp creation without regen

v2writable: avoid mm_tmp creation without regen

Creating mm_tmp is an expensive operation with large inboxes
and can be avoided if there are no new messages to process.

Since git-fetch(1) currently lacks an --exit-code option(*),
mirrors will run `public-inbox-index' unconditionally after
fetch, which is an expensive op if it needs to duplicate
a large SQLite DB.

This speeds up the mirror case of:

	git --git-dir=git/$EPOCH.git fetch && public-inbox-index

This reduces the no-op `public-inbox-index' time from over 8s to
~0.5s on a (currently) 7-epoch clone of https://lore.kernel.org/lkml/
on my system.

(*) WIP --exit-code for git-fetch:
    https://public-inbox.org/git/87ftphw7mv.fsf@evledraar.gmail.com/