ceph-pr-commits: use origin/master as the base commit

ceph-pr-commits: use origin/master as the base commit

instead of using `${sha1}^1...${sha1}^2` use
`origin/master...origin/pr/${ghprbPullId}/head` for getting the range of
commits introduced by a PR. as per `man git-diff`

> "git diff A...B" is equivalent to "git diff $(git-merge-base A B) B".

and `origin/${ghprbTargetBranch}` and `origin/pr/${ghprbPullId}/head` are fetched by
the ghprb-plugin plugin, so it's safe to use them. see
https://plugins.jenkins.io/ghprb/

Signed-off-by: Kefu Chai <kchai@redhat.com>