Blender Git Loki

Fix T63794: duplicating an armature clears B-Bones' custom handle references.

This patch fixes T63794 by updating duplicated bones' custom handle references
to point to the new armature's bones.

The problem occurs because B-Bones of a duplicated armature keep pointing to
the old armature's bones. The references are cleared upon entering Edit Mode
because there are no matching `EditBone` for such bones, and `find_ebone_link`
just returns `NULL` in such cases.

Reviewers: mont29, angavrilov

Maniphest Tasks: T63794

Differential Revision: https://developer.blender.org/D4726