Provide the actual sha for the commit that triggered the build.
While the sha1 variable sometimes gives us the correct information in
the event of an unsuccessful merge, we always need access to the sha for
internal purposes, so we added it as another string parameter to the
build.
Based on farmdawgnation's (https://github.com/farmdawgnation)
pull request (https://github.com/janinko/ghprb/pull/23).