JENKINS-25353: Cleaned up index.lock file when checkout is aborted
Test index.lock removal - JENKINS-25353
Make JGitAPIImpl default behavior match CliGitAPIImpl default

Previously, if JGitAPIImpl was constructed with a null workspace, it
would later throw exceptions. If CliGitAPIImpl was constructed with a null
workspace, it would use the current working directory as the workspace.

Since CliGitAPIImpl is the reference implementation, make JGitAPIImpl
match it.