GitHub submodule links

Hyperlinks submodule hashes and submodule diffs on GitHub.

GitHub doesn't hyperlink to submodules in diffs. To make it easier to track what changed, this extension inserts links to both the referenced submodule commits as well as diffs between them. The same extension for Safari and Firefox as well as source code is available at https://github.com/msolomon/github-submodule-links