Incremental Rewrite of git-submodules

To manage the external dependencies, git submodule commands have been frequently used by the developers in their projects. But as most of the subcommands of git submodule are still written in shell scripts (All, but git submodule init), my project intends to convert the subcommands into C code, thus making them builtins. This will increase Git's portability and hence the efficiency of working with the git-submodule commands. The function cmd_init has been ported to its built-in successfully and similar operation needs to be done to the remaining functions.