list-objects.c: factor out traverse_trees_and_blobs

list-objects.c: factor out traverse_trees_and_blobs

With traverse_trees_and_blobs factored out of the main traverse function,
the next patch can introduce an in-order revision walking with ease.

In the next patch we'll call `traverse_trees_and_blobs` from within the
loop walking the commits, such that we'll have one invocation of that
function per commit.  That is why we do not want to have memory allocations
in that function, such as we'd have if we were to use a strbuf locally.
Pass a strbuf from traverse_commit_list into the blob and tree traversing
function as a scratch pad that only needs to be allocated once.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>