revision walker: mini clean-up

revision walker: mini clean-up

This removes the unnecessary indirection of "revs->prune_fn",
since that function is always the same one (or NULL), and there
is in fact not even an abstraction reason to make it a function
(i.e. its not called from some other file and doesn't allow us
to keep the function itself static or anything like that).

It then just replaces it with a bit that says "prune or not",
and if not pruning, every commit gets TREECHANGE.

That in turn means that

 - if (!revs->prune_fn || (flags & TREECHANGE))
 - if (revs->prune_fn && !(flags & TREECHANGE))

just become

 - if (flags & TREECHANGE)
 - if (!(flags & TREECHANGE))

respectively.

Together with adding the "single_parent()" helper function, the "complex"
conditional now becomes

	if (!(flags & TREECHANGE) && rev->dense && single_parent(commit))
		continue;

Also indirection of "revs->dense" checking is thrown away the
same way, because TREECHANGE bit is set appropriately now.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>