tracker_service: display the Git revision for the current online data

The tracker exposes information from data/CVE/list and other files to the web, through HTML pages and JSON export.

We have some caching support for the JSON export e.g. in lts-cve-triage.py but it is incorrect/inefficient, because it downloads the data whenever there is a new commit, while the security tracker is updated only a few times per hour.

Moreover, when the data is not updated (e.g. due to inconsistent data), it would be informative to know the latest working revision (which the tracker stayed on).

This could be part of the JSON export, or available through other means (new API endpoint, easily extractable info on the frontpage, ...).