Add "shallow_clone" option

This just maps to --depth=1 for git at the moment; I didn't
bother with other version control systems because to be honest
they're basically irrelevant.

Using this option is a *lot* faster to just get an initial
build going.  gdk-pixbuf's git repo is an 100MB download, but only
1.8 currently with depth 1.

https://bugzilla.gnome.org/show_bug.cgi?id=697818