path-walk: support `combine` filter

The `combine` filter takes the intersection of its children, that is: objects are shown only when all child filters would admit the object. The preceding patches added support for many individual filter types. Enable users to compose these filters by implementing support for the `combine` filter type. Mapping intersection onto path_walk_info works because every supported child filter is a monotonic restriction: - `blob:none`, `tree:0` unconditionally clear `info->blobs` and (for `tree:0`) `info->trees`; clearing an already-cleared flag is a no-op. - `object:type=X` is now expressed as an AND of each type flag with the filtered type, so applying multiple such filters only refines the existing set rather than overwrites it. - `blob:limit=N` has to compose too: the intersection of "size < L1" and "size < L2" is "size < min(L1, L2)". Update the `LOFC_BLOB_LIMIT` handler to take the running minimum when `info->blob_limit` is already set, so a combined filter with, e.g., both "blob:limit=10" and "blob:limit=5" produces a limit of 5 regardless of ordering. - `sparse:oid` is left unchanged. A `combine` filter that includes a `sparse:oid` is allowed at most once, since the existing handler refuses to overwrite `info->pl`. Two `sparse:oid` filters in a single `combine` would be unusual and are rejected with a warning, matching the standalone `sparse:oid` behavior. Implementation-wise, the existing `prepare_filters()` called `list_objects_filter_release()` inside each case branch. That works fine for top-level filters, but `combine` filters need to recurse over its child filters without releasing each one in turn (since the parent's release iterates the sub array). Split `prepare_filters()` into a recursive helper that performs only the mutation, plus a thin wrapper that calls the helper and then releases the top-level filter once. The `LOFC_COMBINE` case in the helper just walks `sub_nr` and recurses; child filters are released by the wrapper's single `list_objects_filter_release()` call on the parent (which itself recursively releases each sub-filter, the same way it always has). If any sub-filter is unsupported (e.g. "tree:1", "sparse:<path>", or a not-yet-supported choice), the recursion bubbles a failure up and the existing pack-objects/backfill fallback paths kick in. Add coverage in t6601: - "combine:blob:none+tree:0" collapses to "tree:0" - "combine:object:type=blob+blob:limit=3" yields only the blobs smaller than three bytes - "combine:object:type=blob+object:type=tree" intersects to empty - "combine:tree:1+blob:none" reports the "tree:1" error. Update Documentation/git-pack-objects.adoc to add combine 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>