path-walk: support `tree:0` filter

path-walk: support `tree:0` filter

The `tree:0` object filter omits all trees and blobs from the result,
keeping only commits and tags. Consequently, this filter type should
has a fairly straightforward integration with path-walk, as the decision
to include an object depends only on its type and does not depend on any
path-sensitive state.

Mapping it onto `path_walk_info` is direct: set `info->trees = 0` and
`info->blobs = 0` in `prepare_filters()` when the `LOFC_TREE_DEPTH`
choice is requested with depth zero. The existing code already plumbs
those flags through the rest of the walk:

 - 'walk_objects_by_path()' sets `revs->blob_objects = info->blobs` and
   `revs->tree_objects = info->trees` before `prepare_revision_walk()`,
   so the revision walk doesn't try to enumerate trees or blobs itself.

 - The commit-walk loop short-circuits the root-tree fetch with
   "if (!info->trees && !info->blobs) continue;", so we never even
   look up the root tree, let alone descend into it.

 - `setup_pending_objects()` skips pending trees and blobs based on
   the same flags.

This means the path-walk doesn't allocate or expand any tree structures
at all under `tree:0`, which matches the intended behavior of the
filter.

However, this requires first fixing some issues with how the path-walk
API handles directly-requested trees _and_ trees requested through
lightweight tags. These changes create substantial updates to
t6601-path-walk.sh, which the previous change highlighted as a problem
by tagging otherwise-unreachable trees and having them not appear in the
output.

Non-zero tree-depth filters are not supported. Those depend on the depth
at which a tree is visited, which is a path-walk concept the filter
machinery doesn't currently share with the path-walk API. Reject them in
`prepare_filters()` with a helpful error and let pack-objects fall back
to the regular traversal, the same way it already does for unsupported
filters.

Add coverage in t6601 for both `--all` and a single-branch case to
confirm that no trees or blobs are emitted, and a separate test that
`tree:1` is rejected with the expected error message. Place the new
tests before "setup sparse filter blob" so they run on the original set
of refs, before the orphan branch that the sparse-tree tests create.

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