git-debpush: Defend against git-deborig #931509

git-debpush: Defend against git-deborig #931509

Without this, when git-deborig fails and #931509 isn't fixed,
git-debpush will silently exit with a nonzero exit status which is
very unfriendly.

With this change it does something like this:
  git-debpush: git-deborig failed; maybe try git-debpush --upstream=TAG
  fatal: Invalid object name 'refs/tags/couldn't find any of the following tags'.
It then exits status 127.  This is obviously daft, but making this
any better is even more complex.

When git-deborig is fixed it will do this:
  couldn't find any of the following tags: 1.0, v1.0, upstream/1.0
  tell me a tag or branch head to make an orig.tar from: git deborig --just-print '--version=1.0-1' COMMITTISH
  git-debpush: git-deborig failed; maybe try git-debpush --upstream=TAG
which is still not brilliant but I guess it will do.

The downside, if it is one, is that we lose git-deborig's original
exit status.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
Acked-by: Sean Whitton <spwhitton@spwhitton.name>