path-walk: always emit directly-requested objects

path-walk: always emit directly-requested objects

We are preparing to integrate the path-walk API with some --filter options
in 'git pack-objects', but there is a subtle issue that is revealed when
those are put together and the test suite is run with
GIT_TEST_PACK_PATH_WALK=1.

When a filter reduces the set of requested objects, this results in
filtering out directly-requested objects, such as in the download of needed
blobs in a blobless partial clone.

The root cause is that the scan of pending objects in the path-walk API
respects the filters set in the path_walk_info instead of overriding them
for pending objects.

We can tell that a path is part of the directly-referenced objects if its
path name starts with '/' (other paths, including root trees never have this
starting character). Create a path_is_for_direct_objects() to make this
meaning clear, especially as we add more references in the future as we
integrate the path-walk API with partial clone filter options.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>