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.
Merge commit 'db1ef532b67efbb2d24be4f05e67590d3915400c' into HEAD