Shallow clone/fetch of package source

For automated builds in a CI setup, downloading the entire repository history takes longer and puts more load on the git server -- an option to tell bake to use a depth of 1 for cloning/fetching repositories would be useful.

As a test, cloning a repository the size of ns-3 repository takes 42.241s, with 267.98MiB received. Cloning just the most recent commit for a branch with depth=1 takes 21.876s, with 27.73MiB received.