Throw an exception when the forked git process fails.

This throws an exception when the exit code of the forked git process is nonzero.

There is a bug in Process where redirection-like methods like `#>` discard the exit code of the process.  This is because the methods are implemented on top of piping, which should discard the exit code of the input process.  This is fixed in the sbt 0.13 branch, but until 0.13.1, this means that the JGitRunner will not fail when the forked process fails.