fast-import fails in read-only tree

I recently discovered that "git fast-import" signals an error if used in a tree to which we do not have write-access, because it tries to create a "objects/pack/tmp_pack_XXX" file even before starting to process the commands.

Usually this is not a problem (we'll create new commits and such, so write-access is indeed necessary), but in my case I was using fast-import only for its "reading" operations (in order to combine several inter-dependent "cat-file" operations into a single git session).

        Stefan