test suite: t-make-badcommit: Generate a commit we can fix up

This is very like a commit generated by #849041.  The wrongness is the
same.  The difference is that it's not in a dgit-generated merge, but
the fixup script doesn't care about that.

The existing test case which uses this (drs-push-rejects) still sees
badness and is therefore still happy.

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