Copy submodule info into ".git/config" file.
Just like "git submodule init", this copies information about the submodule into ".git/config". You can use the accessor functions above to alter the in-memory git_submodule object and control what is written to the config, overriding what is in .gitmodules.
By default, existing entries will not be overwritten, but setting this to true forces them to be updated.
0 on success, < 0 on failure.