[PATCH 0/2] Teach "stg delete" to delete several patches at once

The first patch introduces a test for "stg delete", since there wasn't already one, and it's much less stressful to modify stuff if you have an automatic test that can tell you if you broke stuff.

As it turns out, this test finds an existing bug: it's not possible to delete a patch in another branch if there's an identically named branch in the current branch, and both patches are topmost in their respective applied stacks.

The second patch teaches "delete" how to delete multiple patches in one go.