Automate the re-fetching of code-intel-extensions for add-on bundle if out of date

Created by: marekweb

When we fetch and bundle code intel extensions for the Firefox add-on, the build script takes a the revision to fetch from the config in bundled-code-intel-extensions.json.

In order to avoid re-fetching the repository on every build, the build script assumes that if a copy of the repo exists, then it doesn't need to be refetched.

The problem is that when we change the revision in the config file, the script isn't aware that it should re-fetch the repo.

The solution is to check the current revision of the copy of the repo (.e.g git rev-parse) and compare it to the one which is specified in the configuration. If it doesn't match, then the copy should be deleted and re-fetched.