The rollback functionality in
services/repository/repository.go:ForkRepository is incorrect and could
lead to a deadlock as it uses DeleteRepository to delete the rolled-back
repository - a function which creates its own transaction.

This PR adjusts the rollback function to only use RemoveAll as any
database changes will be automatically rolled-back. It also handles
panics and adjusts the Close within WithTx to ensure that if there is a
panic the session will always be closed.

Signed-off-by: Andrew Thornton <art27@cantab.net>