Cope when cloning suite which doesn't receive uploads, like testing.

Cope when cloning suite which doesn't receive uploads, like testing.

If $lastpush_hash is '', it means that the dgit git server does not
have the suite branch.  That can even happen if the current version in
the suite was a .dsc with a Dgit field, if an in-archive copy moved
the .dsc from whereever it was uploaded.

Do not crash in this situation.  Instead, give the user the Dgit hash
(and maybe pseudomerge it with the user's history - sigh).

There is still a possible problem: maybe the referenced commit is on
another branch.

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