Set single class for stale/good

- Before this was adding a class each time, so colours would fail to update
- Now just sets a single class, which updates the colour correctly
Remove oldest commit time if doesn't exist
Merge pull request #3 from evilstreak/fix-stale-display-bugs

Fix stale display bugs
Retitle oldest commit as oldest merge

Commit e3bef0d updated the timestamp to show the age of the oldest merge
rather than the oldest commit. This updates the title to match.
Merge pull request #4 from evilstreak/update-oldest-commit-title

Retitle oldest commit as oldest merge
Extract update_repo method
Rename build_api_url to build_api_compare_url

We want to add a call to another API endpoint, so make the existing
method name a bit more specific.
Move api_compare_url and http_compare_url to object
Split repo update into an update and redraw method

As well as cleaning up the split between code which is working things
out based on the API response and code which is rendering stuff to the
view, this lays the groundwork for calling redraw independently of
updating the repo from an API call to the compare endpoint.
Cleanup formatting
Use tags when linking to Github compare view

After updating the repo state from the compare endpoint, grab the sha of
the two ends of the compare, fetch the list of tags for the repo, look
up the first tag for each of the shas, and use those tags to build the
compare URL linked to from the title.

Our use case for this tracker is to compare the difference between a
branch that tracks what's been deployed to production and master.
Because these branches are moving targets, when we prepare a release
candidate for a deploy we link to specific commits which are tagged with
(never updated) release_123 type tags. Using those tags to build the
compare URL makes it one-click easy to get a link for a release
candidate.
Turn tag resolving on with a query parameter

Set resolve_tags to anything except an empty string to look up tags for
the start and end commits of the compare and use those in the compare
link.