walk_object: ignore blob content if not requested

walk_object: ignore blob content if not requested

Previously, walk_object would read the contents of each object it
encountered, even if it subsequently threw away those contents.

With this change, it will skip reading the contents of blobs unless
the caller explicitly asked for blob contents. In particular, this
means operations that traverse the object graph without caring
about object contents, such as `bup gc`, now only need to read the
contents of tree and commit objects.

Tested: `make test`; local run against copy of production repo.
Signed-off-by: Ben Kelly <btk@google.com>
[rlb@defaultvalue.org: shorten commit summary; adjust indentation;
 adjust docstring and comment; test "not include_data" first; change to
 type='blob']

Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>