Merge lp://staging/~gz/bzr-builddeb/merge_changelog_invalid_version_893495 into lp://staging/bzr-builddeb

Handle dpkg-mergechangelogs aborting by falling back to other merge hooks rather than thinking it's a successful merge resulting in a zero byte file. This can be hit with invalid versions in one of the changelogs, and newer dpkg versions have become stricter on this front.

Also resolved some bad encoding handling while there, as it was part of the original bug report and gets in the way of one of the neater way of testing (non-ascii versions are rejected in my older dpkg version as well).

The test is written in such a way as it should keep passing if my dpkg patch is accepted and the merge starts succeeding it will continue to pass, regardless of the version installed. That's the plan, at least.