Support for running "git -C" to run git commands within a specified directory

[sirnewton01]

The standard git command supports it and it can be really useful to run commands in another git repository without having to chdir.

[driusan]

The git man page says it can be specified multiple times, but the Go flag package only let's us specify flags once. (Some other subcommands options have this problem too.)

Regardless, I can't imaging any reason you'd want to, so I think we can call this resolved..