pack-objects: support `--delta-islands` with `--path-walk`

Since the inception of `--path-walk`, this option has had a documented incompatibility with `--delta-islands`. When discussing those original patches on the list, a message from Stolee in [1] noted the following: this could be remedied by [...] doing a separate walk to identify islands using the normal method In a related portion of the thread, Peff explains[2]: The delta islands code already does its own tree walk to propagate the bits down (it does rely on the base walk's show_commit() to propagate through the commits). Once each object has its island bitmaps, I think however you choose to come up with delta candidates [...] you should be able to use it. It's fundamentally just answering the question of "am I allowed to delta between these two objects". That is similar to what this patch does, and it turns out the cheaper option is sufficient: perform the same island side effects from the path-walk callback rather than doing a second walk. Recall how delta-islands are computed during a normal repack: - `show_commit()` calls `propagate_island_marks()` for each commit, which merges the commit's island bitset onto its root tree object and onto each of its parent commits. - `show_object()` for a tree records the tree's depth derived from the slash-separated pathname. Subsequent `resolve_tree_islands()` uses that depth to walk trees in increasing-depth order, propagating each tree's marks to its children. - At delta-search time, `in_same_island()` enforces that a delta target's island bitmap is a subset of its base's: every island that reaches the target must also reach the base. Path-walk's enumeration callback is `add_objects_by_path()`. It already adds objects to `to_pack`, but until now did not perform the island-related side effects. Two things are needed: - For each commit batch, call `propagate_island_marks()` on commits, exactly as `show_commit()` does. We have to be careful about the order in which we call this function, and we must see a commit before its parents in order to have island marks to propagate. The path-walk batch preserves that order. Path-walk appends commits to its `OBJ_COMMIT` batch as they come back from the same `get_revision()` loop the regular traversal uses, and `add_objects_by_path()` iterates the batch in array order. So every commit reaches `propagate_island_marks()` in the same sequence that `show_commit()` would have seen it, and the descendant-first chain that the algorithm relies on is intact. Skip island propagation for excluded commits to match the regular traversal, whose `show_commit()` callback is only invoked for interesting commits. Boundary commits may still be present in path-walk's callback so they can serve as thin-pack bases, but they should not contribute island marks. - For each tree batch, record the tree's depth from the path. Use the `record_tree_depth()` helper from the previous commit so both callbacks behave identically, including the max-depth-wins behavior when a tree is reached via more than one path. The helper accepts both the `show_object()` path shape ("foo", "foo/bar") and the path-walk shape with a trailing slash ("foo/", "foo/bar/"), so depths recorded from either traversal mode are directly comparable. This is implicit in the implementation sketch from Peff above. `resolve_tree_islands()` sorts trees by `oe->tree_depth` in increasing-depth order before propagating marks down, so that a parent tree's marks are finalized before its children inherit them. Without recording the depth at path-walk time, every path-walk-discovered tree would land at depth 0 in `to_pack`, the sort would lose its ordering, and children could inherit marks from parents whose own contributions had not yet been merged in. With those two pieces in place, `resolve_tree_islands()` receives the same island inputs from path-walk as it would from the regular traversal, so the existing island checks can be reused unchanged. Drop the documented incompatibility between `--path-walk` and `--delta-islands`, and add t5320 coverage for path-walk island repacks with and without bitmap writing, as well as the same-island case where a delta remains allowed. [1]: https://lore.kernel.org/git/9aa2471b-0850-4707-9733-d3b33609f5f2@gmail.com/ [2]: https://lore.kernel.org/git/20240911063203.GA1538586@coredump.intra.peff.net/ Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>