tests/test-rollback

patchbomb: fix bug introduced in 4ddfad7ebd98 and add test

Thanks to Augie Fackler for reporting this.

#!/bin/sh

mkdir t
cd t
hg init
echo a > a
hg add a
hg commit -m "test" -d "1000000 0"
hg verify
hg parents
hg status
hg rollback
hg verify
hg parents
hg status

echo % Test issue 902
hg commit -m "test"
hg branch test
hg rollback
hg branch

echo % Test rollback of hg before issue 902 was fixed
hg commit -m "test"
hg branch test
rm .hg/undo.branch
hg rollback
hg branch