dgit: Internal change: Support forcing split source builds

dgit: Internal change: Support forcing split source builds

We are going to start handling weirder kinds of git tree, which will
require a kind of `split brain': the dgit-using maintainer's view will
be a git tree which is not a dgit git tree.  dgit will convert them
during push.

For this to work we will have to have dgit always (in these cases)
generate the source package itself (with a separate invocation of
dpkg-source).  This will involve some dis- and re-entangling of the
way we generate arguments to dpkg-buildpackage.

We can profitably split this up into this pre-patch, which has no
overall functional change for normal users.

We do here provide a new --always-split-source-build option which
allows the new approach to be explicitly requested.  This allows us to
use the test suite to not only test that we didn't break any of the
existing building strategies, but also that the new strategies all do
what we expect.

But this option is not really semantically useful for users so we do
not document it.