add git-fix-modules.sh and step-by-step tutorial

[shamrin]

• Add ./git-fix-modules.sh, a more helpful replacement for git submodule update --init.
• Add TUTORIAL.md
• Add test

TODO for later PRs:

• [ ] remove url = . suggestion from README.md
• [ ] put TUTORIAL.md content into README.md
• [ ] check ./git-fix-modules.sh with non-main repos
• [ ] install ./git-fix-modules.sh into $PATH
• [ ] change t/test.sh so that it actually tests something

Fixes #7, fixes #4.

P.S. ./git-fix-modules.sh helper script was written by @apenwarr. I did clean it up a bit. If there's anything broken with it, it's my fault.