update_conf: strip trailing whitespace from blame revision

Created by: craftyguy

git python cannot handle trailing whitespaces, which are preserved after the split('=') and passed as a part of the sha to gitpython

I wasn't able to test this in CI since we don't have a way to specify custom mesa_ci branches for the 'accept failured tests' build. This is meant to prevent this failure: http://otc-mesa-ci.jf.intel.com/job/accept_failed_tests/1117/console