Default to command-line git, instead of JGit

[eed3si9n]

Branching off from #243 (comment).

Let's default to using the command-line git instead of using an emulation via JGit.
There's been issued raised around the work tree support, and workarounds are proposed to automatically change the behavior around this #243, but my guess is that the need for JGit is limited to the default. The popularity of sbt-dynver sort of is an existential proof that most developers working on open source would have git installed on PATH.