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>

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