Test that pulls a patch creating a file that got modified afterwards

This demonstrates an issue wite has bitten me more than once: the stg
branch adds a file in one patch, and modifies it in a later patch; then all
patches get integrated in upstream tree, and at "stg pull" time, stgit
believes there is a conflict, even when the patches are exactly the same.

This is normal as it requires the --merged flag on push or pull.  So
we rollback with "push --undo" and "push --merge" to finish.