build: add --fetch-only option

It should be possible to do:

git ubuntu clone <srcpkg>
cd <srcpkg>
... make upstream change ...
dpkg-source --commit

But the last step fails because the orig tarball doesn't exist yet. While our internal changelogify logic in the newest branch can generate this for us, the above should also be nicely wrapped.

In that branch, we added a fetch_orig() standalone helper function for use by changelogify, let's just add a helper for that.

Perhaps it should be top-level? (e.g. git ubuntu export-orig) ?