git-apply: parse the whole list of patches into memory first

git-apply: parse the whole list of patches into memory first

Make it a clear two-phase thing: first a read-only parse of
the patch itself (which is independent of any current index
information), and then the second phase actually uses the patch.

The second phase might not be a real apply, it could be just a
diffstat, for example. Which is trivial to do once the patch is
parsed.