dgit: Do not misrecognise some initial packaging as gdr-processable

dgit: Do not misrecognise some initial packaging as gdr-processable

git-debrebase expects a breakwater start commit to not make any
changes to upstream files.  This means that sometimes dgit would fail
to make this check, and expect gdr to be able to make patches; but
then gdr would reject the branch.

It is still of course possible for dgit to ask gdr to make patches for
branches that the user does not *intend* to be handled by gdr, but
that is OK so long as gdr will DTRT.

Closes: #922446
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>