`init` command in Go.

Open questions

 * How to do the config checking? Do we want to parse the config files in init
 and check for proper values? Or do we want to just check if the files exists
 and yell at user if the config is missing when they invoke a command that uses
 that config?

Change-Id: 5ab51c5e33
Story-Id: 79247470
Would be nice to perhaps put this into a package on its own, let's say `git-trunk/shell/Run()`...