Update llvm bot monitor for git hashes

- The revisions are now git hashes, so don't try to parse them as
  integers.
- Truncate hashes to 8 characters for user-facing display.
- The JSON object returned by buildbot has the commits in time order, so
  we don't need to iterate the whole list to find the first and last
  ones.

Change-Id: I818f36a65b85b23a64017106494e56141095966e
1 file changed