[RFC PATCH 6/7] pack-objects: extract `record_tree_depth()` helper

[Taylor Blau <me@ttaylorr.com>]

Prepare for a future change that needs to record tree depths from a
second call site by factoring out the delta islands-specific portion of
`show_object()` out into a helper, `record_tree_depth()`.

`record_tree_depth()` takes a tree OID along with the path that
`show_object()` received, and computes the directory depth from the
slash count in the path.

While we're in the area, make a few minor clean-ups:

 - Gate the call on `obj->type == OBJ_TREE`, as we only care to compute
   the depth for tree objects. The sole caller of `oe_tree_depth()`
   resides in `delta-islands.c::resolve_tree_islands()`, and only calls
   `oe_tree_depth()` behind `oe_type(...) == OBJ_TREE`.

 - Defer computing the depth for an object until we know it is in the
   `to_pack` list.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 builtin/pack-objects.c | 34 ++++++++++++++++++++--------------
 1 file changed, 20 insertions(+), 14 deletions(-)

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 1a5f1afd32e..842d1fcac29 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -2722,6 +2722,24 @@ static inline void oe_set_tree_depth(struct packing_data *pack,
 	pack->tree_depth[e - pack->objects] = tree_depth;
 }
 
+static void record_tree_depth(const struct object_id *oid, const char *name)
+{
+	const char *p;
+	unsigned depth;
+	struct object_entry *ent = packlist_find(&to_pack, oid);
+
+	if (!ent)
+		return;
+
+	/* the empty string is a root tree, which is depth 0 */
+	depth = *name ? 1 : 0;
+	for (p = strchr(name, '/'); p; p = strchr(p + 1, '/'))
+		depth++;
+
+	if (depth > oe_tree_depth(&to_pack, ent))
+		oe_set_tree_depth(&to_pack, ent, depth);
+}
+
 /*
  * Return the size of the object without doing any delta
  * reconstruction (so non-deltas are true object sizes, but deltas
@@ -4375,20 +4393,8 @@ static void show_object(struct object *obj, const char *name,
 	add_preferred_base_object(name);
 	add_object_entry(&obj->oid, obj->type, name, 0);
 
-	if (use_delta_islands) {
-		const char *p;
-		unsigned depth;
-		struct object_entry *ent;
-
-		/* the empty string is a root tree, which is depth 0 */
-		depth = *name ? 1 : 0;
-		for (p = strchr(name, '/'); p; p = strchr(p + 1, '/'))
-			depth++;
-
-		ent = packlist_find(&to_pack, &obj->oid);
-		if (ent && depth > oe_tree_depth(&to_pack, ent))
-			oe_set_tree_depth(&to_pack, ent, depth);
-	}
+	if (use_delta_islands && obj->type == OBJ_TREE)
+		record_tree_depth(&obj->oid, name);
 }
 
 static void show_object__ma_allow_any(struct object *obj, const char *name, void *data)
-- 
2.54.0.4.g6aa0d38a4ec