transactions: avoid late tear-down (issue641)
We use weak references (ugh) to avoid having to manually delete
transaction references out of each call frame when an exception occurs.
adding dir/bar
adding foo
dir/bar
foo
adding dir/bar_2
adding foo_2
dir/bar_2
foo_2
adding a
adding c
adding b
adding d
removing a
removing c
recording removal of a as rename to b (100% similar)