prscript: display url where user can watch build
prscript: support bigger PR

The script now looks for originan HEAD in 100 commits instead of 30.
It should be enough becasue a sane PR should not have 100 commits.
prscript: update code following buildbot upgrade

The authentication scheme did change on the buildbot due to a
software upgrade. This patch update prscript.py to fix the build
submission.