Local git instead of go-git

• Stops using go-git for running git commands, and instead rely on local installation of git.
• Add optional argument to git-walk to start <commit>, so one can start navigating from a specific commit, instead of being forced to start from the oldest available commit.

First release

Basic support for git-walk to [start|next|end]. Saves reference in ../.git-walk file (saving in the parent directory is something to be improved, but necessary for the moment).