Fixed cwd for the VCS cmd

VCS `cmd` should run from the `baseDir` of this VCS
- because then you relativize paths of added/commited files to `baseDir` ([here](https://github.com/sbt/sbt-release/blob/master/src/main/scala/ReleaseExtra.scala#L125-L127) for example)
- if you clone other git repo inside of the current one and try to use it with `Git.mkVcs(file("another-repo"))` it doesn't work (runs from the current repo's `baseDir`)

So I just added the cwd parameter to the [`Process`](http://www.scala-lang.org/api/2.10.3/index.html#scala.sys.process.Process$) constructor.