Add support to BitBucket archive for commit hash fragments in the URL

[philipstarkey]

Currently to view details about a commit in the BitBucket archive, you need the full changeset hash. This differs from BitBucket (and GitHub) where you only need the first N characters of the hash in order to look it up. This means that some links in the archive (to commits) my not work (although most do because we follow all links, which means we download multiple copies of the commit data for each hash fragment we find....but this only catches URLs to hash fragments, not floating changeset fragments)

We need a way of translating this fragment into the full hash so we can look up the correct JSON file, and then to implement this in the AngularJS template