prscript: add --norebase option

If --norebase option is provided, the prscript will start a build
that can be used to check if an out-of-sync branch pass the test.

PR builds:
- PR build: https://buildbot.suricata-ids.org/builders/regit/builds/82
- PR pcaps: https://buildbot.suricata-ids.org/builders/regit-pcap/builds/21