suite: Speed up 'newest' feature

suite: Speed up 'newest' feature

For a 'newest' value of N, we were calling find_git_parent N times.
find_git_parent was invoking githelper's 'refresh' endpoint each time,
which in my testing took 20-30 seconds. So, for a sha1 that needed 20
backtracks, I was seeing teuthology-suite take eight minutes to
complete. With this change, it took 30 seconds for the same
teuthology-suite invocation.

We were also not including the trailing slash on the 'refresh' endpoint,
resulting in a 308 redirect each time - doubling the request count.
That's fixed as well.

Signed-off-by: Zack Cerza <zack@redhat.com>