dgit: Do not generate bogus commits

dgit: Do not generate bogus commits

There were two places in the code where ill-formed commits were
generated: both psedumerges.

One is in dgit import-dsc, a fairly minor problem.

The other is in --overwrite and is very bad because that's an
important option.

Nothing in git seems to notice, unless you run git-fsck.  Even a git
server does not, by default.  However, some other popular git servers
do reject these broken commits.

I have tested this changes with git-fsck (by using the test suite
patches which follow in this series) and now everything seems fine.

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