path-walk: add pl_sparse_trees to control tree pruning

The path-walk API prunes trees and blobs when a sparse-checkout pattern list is provided, which is the correct behavior for 'git backfill --sparse' since it only needs to fill in objects at paths within the sparse cone. However, a future change will use the path-walk API with a sparse:<oid> filter that restricts only blobs while retaining all reachable trees. To support both behaviors, add a 'pl_sparse_trees' flag to path_walk_info. When set (as in 'git backfill --sparse' and the --stdin-pl test helper mode), the sparse patterns prune both trees and blobs. When unset, only blobs are filtered and all trees are walked and reported. Additionally, move the SEEN flag assignment in add_tree_entries() to after the sparse pattern and pathspec checks. Previously, SEEN was set immediately upon discovering an object, before checking whether its path matched the sparse patterns. When the same object ID appeared at multiple paths (e.g. sibling directories with identical contents), the first path to be visited would mark the object as SEEN. If that path was outside the sparse cone, the object would be skipped there but also never discovered at its in-cone path. By deferring the SEEN flag until after the checks pass, objects that are skipped due to sparse filtering remain discoverable at other paths where they may be in scope. Signed-off-by: Derrick Stolee <stolee@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>