JENKINS-24728 Added UI Elements for shallow clone depth

[nastharl]

This is based off some old code, and i saw final got added to a lot of parts of the clone option. Not sure if i need to rework this due to that or not.

[jenkinsadmin]

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[MarkEWaite]

@nastharl would you be willing to review 2de79b8b552b7930adc34bf527743f8abd30860f ? I spent a day exploring various alternatives and simplifications and would like your insights if you can detect issues with the changes I made.

Specifically, I'd like your insights on the JGit incompatibility which seems to result from the simplified implementatoin. Previously, JGit would warn that shallow clones are not supported, and would perform a full depth clone. The current implementation on 2de79b8b552b7930adc34bf527743f8abd30860f causes JGit to throw an error that shallow clone is not supported. I suspect that is because I've placed the depth setting on the fetch operations, not just on the clone operations.

I also implemented an automated acceptance test.

[MarkEWaite]

A rebased and modified form of this commit was merged to the master branch as d96318d