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.
#!/bin/sh
mkdir a
cd a
hg init
echo foo > b
hg add b
hg ci -m "b" -d "1000000 0"
chmod -w .hg/store
cd ..
hg clone a b
chmod +w a/.hg/store # let test clean up
cd b
hg verify