Check for git submodule --recursive support

[nilbus]

git submodule x --recursive is only supported after git 1.6.5.
See https://github.com/git/git/blob/master/Documentation/RelNotes/1.6.5.txt#L147

Users with older versions of git on the deploy target using remote-cache or similar deploy methods would see this error when deploying:

 ** [10.0.0.1 :: err] Usage: git submodule [--quiet] [--cached] [add [-b branch] <repo> <path>]|[status|init|update [-i|--init] [-N|--no-fetch]|summary [-n|--summary-limit <n>] [<commit>]] [--] [<path>...]|[foreach <command>]|[sync [--] [<path>...]]
    command finished in 27929ms
*** [deploy:update_code] rolling back

This fix was tested on an OSX 10.6 (git 1.7.4.1) and Ubuntu Linux 9.10 (git 1.6.3.3) deploy targets, and against both GNU bash and BSD dash (sh).