Watch Git's lockfile for safer updates

[thomaswilburn]

When a repo gets pulled, especially if there's a rebase going on and there's a graphic page loaded, it's possible to crash the rig while it tries to access watched files that aren't available temporarily. We also don't watch the .git directory, since we don't want to bog that down. However, if we watch .git/index.lock, we can pause the live reload during Git operations, and then resume once it's done, we just have to work out how to add that watcher without monitoring the entire directory.