Blender Git Loki

Bugfix [#27990] Merge Bones freezes Blender

Recoded side-chain reparenting step to fix (as far as I've been able
to tell) infinite looping problems which were a bit intermittent here
using the test file. The fix here involves some tighter checks to
prevent corrupting the parenting of bones in the run of bones being
merged but also of any ancestors of those.