Add support for specifying Git working dir

[heyblinken]

Can you please add support to be able to optionally pass in the desired Git working directory instead of automatically determining it? I am writing a plugin that allows you to double click on the hash from your "Git Log Graph" scratch window, which then will show a popup of various other git commands you can run. The issue occurs because i have git repos inside of the parent repo ( child repos are ignored in parent repo ) and if i run any of your git commands when on the "Git Log Graph" view when it was generated from one of the child git repos, it will use the parents working dir instead of the childs git repo cwd.

example:
self.view.run_command( "git_add_choice" , { "working_dir":<child_cwd> } )