git-subtree: Added initial support for the 'git subtree add' command.

Signed-off-by: John Butterfield <johnb003@gmail.com>
Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Added support for 'git subtree push', and 'git subtree pull'.

Signed-off-by: John Butterfield <johnb003@gmail.com>
Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Converted some strings to resources and general code cleanup

Signed-off-by: John Butterfield <johnb003@gmail.com>
Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>