Test that stg coalesce handles head != top gracefully

It currently doesn't quite: it will roll back the transaction just
fine, but not before asking the user for a commit message which is
lost in the rollback.

Bug discovered by Erik Sandberg <mandolaerik@gmail.com>:
https://gna.org/bugs/?12204

Signed-off-by: Karl Hasselström <kha@treskal.com>