Allow using either HTTPS or SSH urls

Haven't been able to test because of 404s on the mock-res dependency, so
this probably doesn't work in its current state

refs #1 
Missing "link" here.

I've made a bit of use of these links in the log.  It's not hard to copy the sha and and write the URL to do the same, but the whole reason for this function was to have a bit more convenience.

One alternative would be to unconditionally `assertValidHttpsUrl(push.repository.url)` above.  Valid push events from GitHub should always have this.  Then we could use `push.repository.url` with impunity elsewhere.