pack-objects: extract `record_tree_depth()` helper

pack-objects: extract `record_tree_depth()` helper

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

The helper looks up the object in `to_pack`, returns early when the
object was not added there, computes the depth from the slash count in
the supplied name, and preserves the existing max-depth-wins behavior
when a tree is reached by more than one path.

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