Add the gitsh.gitCommand git-config option

This option is used to specify what command gitsh should use to execute git
commands. When this option is specified using git-config it is overridden by
the command passed using the --git command line option when starting gitsh. The
gitsh.gitCommand variable can also be overridden inside a gitsh session by
using the :set command.

Fixes #111
Test git interaction inside a temporary home dir