Build compare url with tags

Build the compare URL (which is used as the href for the repo title anchor) using tags.

In our workflow tags are stable, whereas the tracked branches keep moving. Having compare URLs which use stable tags means we can use them as release candidates without having to manually lookup the release number tags for the start and end commits.

If tags can't be found for the commits it just falls back to the tracked branches.

The functionality is hidden behind a query parameter – stick `resolve_tags=yep` or any other value you feel like onto the URL to see it.