Azure: Ensure we do a shallow git clone

Azure: Ensure we do a shallow git clone

In Azure, older pipelines such as ours do not default to a shallow fetch
but rather do a complete clone. This introduces a marginal time increase
in each task, but also more importantly takes up significant disk space.
We are now getting warnings in some cases about using more than 95% of
our available disk space so take this as a first easy step to resolve
that problem.

Signed-off-by: Tom Rini <trini@konsulko.com>