Git plugin UI transition for shallow clone needs more changes

jsoref and timja provided a pull request to make the git plugin better fit with upcoming user interface changes.

Unfortunately, the shallow clone depth setting is much less clear than it was before. Previously, the shallow clone depth setting was included in the clone options as:

With the changes that are merged for inclusion in git plugin 4.3.0, it looks like this:

We need to find a way to label the shallow clone depth field. It should probably look something like this when not selected (example from freestyle job configuration page):

and might look something like this when selected:

I'm also open to other recommendations for appearance. I'm especially open to pull requests proposing to improve it.