Lookup submodule information by name or path.
Given either the submodule name or path (they are ususally the same), this returns a structure describing the submodule. If the submodule does not exist, this will return GIT_ENOTFOUND and set the submodule pointer to NULL.
The name of the submodule. Trailing slashes will be ignored.
0 on success, GIT_ENOTFOUND if submodule does not exist, -1 on error