Re: [PATCH] merge: refuse to create too cool a merge by default

[Junio C Hamano <gitster@pobox.com>]

Linus Torvalds <torvalds@linux-foundation.org> writes:

> ... And I guess it might not be too nasty
> to add: it could be done as part of the object checking pass after
> downloading the pack. Was that what you were thinking of?

Not that fancy, actually.  Running an equivalent of

    git rev-list --max-parents=0 ^HEAD FETCH_HEAD

was what I had in mind.  This shouldn't be too costly for a normal
case (O(N) where N is the number of changes on FETCH_HEAD since it
forked from you).

It needs to be done even when FETCH_HEAD is a descendant of HEAD,
(i.e. when we would have fast forwarded without creating a merge) to
be effective, as --no-allow-new-root is about not trusting your
subsystem people not doing silly things on purpose, unlike our
original patches.