Support environment option for setting a git depth for all checkouts

[mauritsvanrees]

In CI it would be useful to do something like this:

GIT_CLONE_DEPTH=1 make test

This would set the git clone depth for each checkout, unless a package has set its own depth in mx.ini.

I can have a try if I have time still today.

[mauritsvanrees]

Fixed in PR #48, released in mxdev 4.1.0.