ceph-pr-commits: Fix check with many-commit PRs

ceph-pr-commits: Fix check with many-commit PRs

Relevant changes:
- All non-compliant commits are reported (previously, only the first
    wrong one was reported, stopping further processing).
- Signed-off-by regex put in place to check for
    <something@somewhere.tld> (including lt and gt symbols). This rule
    could be relaxed, as some commits in the past were sent without
    the enclosing <...>
- `extract_sha` function has been removed

Fixes: http://tracker.ceph.com/issues/36082
Signed-off-by: Ernesto Puerta <epuertat@redhat.com>