The internal btree node traversal here is a bit different than in
dbgbtree.py, dbgmtree.py, etc, because we want to include both inner and
leaf nodes. We could decode the branch tags multiple times, but checking
for bid changes is a bit simpler.
It's interesting to note this went missed for so long, because, well,
it's hard to actually have more than one btree node. And the tests
explicitly covering large btree structures, test_btree, aren't parsable
by dbgbmap.py since they don't create real filesystem images.
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