Extension to "commits since" badge: allow counting from latest version tag

Many projects don't use Github Releases to track releases, but simply ordinary git tags. These show up in the GitHub releases page (for example: https://github.com/Lucretiel/joinery/releases), but aren't marked as releases in the API. It'd be nice if there was another endpoint– perhaps `/commits-since/USER/REPO/latest-tag.svg`– which counts commits after the most recent tag matching `/\d+(\.\d+)/`

Start with the existing “commits since” endpoint and the GitHub releases and tags behavior described in the issue. Trace how the current endpoint determines its reference point, then verify the latest matching version tag and commit count behavior. Done means the proposed latest-tag endpoint is available and its behavior is covered by tests.

Written by the indexing model from the issue text.