pseudomerge: Suppress checks in split brain mode with --overwrite

If the user has specified an appropriate --overwrite (=VERSION with
the archive's version, or without a version and the archive's version
being in debian/changelog), do not do all the automatic checks.

Instead, in this case, simply make the dgit view be a descendant of
the archive, as instructed.

Closes:#841101.

When generating split brain pseudomerge with --overwrite

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>