Test if a path is a submodule.

* bin/git-is-submodule: It's an internal command.
Rename git-dirty-submodule -> git-is-dirty-submodule.

* bin/git-dirty-submodule: From here...
* bin/git-is-dirty-submodule: ... to here.
* bin/git-sb-commit: Adjust.
Pretty commit the submodule(s) given as arguments.

* bin/git-commit-sb: Here.