Allow experimentally to specify GIT_DEPTH

Merged Kamil Trzcińśki requested to merge support-git-depth into master

This allows to shallow clone repository. The value is passed to git fetch and git clone.

It doesn't introduce any new syntax, it only requires to pass GIT_DEPTH variable to build environment. I think of this as an experimental feature that we can later extend with .gitlab-ci.yml syntax:

  GIT_DEPTH: "20"

@tmaczukin What do you think?