path-walk: support `object:type` filter

path-walk: support `object:type` filter

The `object:type` filter accepts only objects of a single type; it is
the second member of the object-info-only filter family that bitmap
traversal already supports.

Like `blob:none` and `tree:0`, it can be evaluated with nothing more
than the object's type, which is exactly the granularity path-walk's
existing info->{commits,trees,blobs,tags} flags already control.

Map `LOFC_OBJECT_TYPE` in `prepare_filters()` by AND-ing each flag
against the filtered type. A single `object:type=X` filter
applied to the default info (all flags = 1) leaves `info->X = 1` and
all the others 0, which is what we want.

Using an AND rather than straight assignment prepares us for a
subsequent change to implement combined object filters.

The path-walk machinery is mostly already wired for the per-type
distinction:

 - `walk_path()` calls `path_fn` for a batch only when the corresponding
   `info->X` flag is set, so unwanted types are silently not reported.

 - `add_tree_entries()` skips tree entries of type `OBJ_BLOB` when
   `info->blobs` is unset, so we don't even allocate paths for them.

 - The commit-walk loop short-circuits the root-tree fetch when
   `!info->trees && !info->blobs`, so commit-only filters don't descend
   into trees at all.

But there are a couple of side effects of the "trees off, blobs on" case
that need fixing:

 1. 'setup_pending_objects()' previously skipped pending trees as soon
    as `info->trees` was zero. For 'object:type=blob' the call site
    needs those pending trees: a lightweight tag pointing to a tree, or
    an annotated tag whose peeled target is a tree, can both reach
    blobs that are otherwise unreachable from any commit's root tree.
    Loosen the gate to "if (!info->trees && !info->blobs) continue" and
    similarly retrieve the root_tree_list whenever either trees or
    blobs are wanted.

 2. The revision machinery's `handle_commit()` drops pending trees when
    `revs->tree_objects` is zero (see the 'OBJ_TREE' handler in
    revision.c), so by the time path-walk sees the pending list
    after `prepare_revision_walk()` the tree-bearing pendings would
    already be gone. Fix this by setting

        revs->tree_objects = info->trees || info->blobs

    so pending trees survive `prepare_revision_walk()` whenever we
    need to walk into them. Path-walk still resets tree_objects to
    zero immediately after `prepare_revision_walk()` returns, so the
    rev-walk itself never enumerates trees redundantly with
    path-walk's own descent.

Add coverage in t6601 for each of the four `object:type` values. The
'object:type=blob' test in particular asserts that file2 and child/file
(both reachable only through tag-pointed trees) show up in the output,
exercising the pending-tree fix.

Update Documentation/git-pack-objects.adoc to add object:type to
the list of supported --filter forms.

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>