Increase test coverage

[MarkEWaite]

Increase test coverage

The increases are minor changes that cover a few more JGit methods without significantly increasing test time.

• Call prune() instead of prune(true) at least once
• Test null depth for shallow fetch
• Test shared() in addition to shared(true)
• Call tags(boolean) from a test
• Test push.force()

Checklist

• I have read the CONTRIBUTING doc
• I have referenced the Jira issue related to my changes in one or more commit messages
• I have added tests that verify my changes
• Unit tests pass locally with my changes
• I have added documentation as necessary
• No Javadoc warnings were introduced with my changes
• No spotbugs warnings were introduced with my changes
• I have interactively tested my changes

Types of changes

• Tests