Git clone depth

[Andykl]

This PR adds specialised Context.clone method and applies it to all the places build system does the clone. That reduces download size a lot (about 99% for mingw-python case).

Use of git clone --revision SHA requieres git at least 2.49 which is not latest default for ubuntu and would also requiere

For Ubuntu, this PPA provides the latest stable upstream Git version

# add-apt-repository ppa:git-core/ppa
# apt update; apt install git

but libyuv and pyobjus are not that big, so that two repos could be reverted to use full clone.

[Andykl]

That fixes #179

[Andykl]

I reverted those couple repos and it should be safe to merge.