Add createAbsoluteSourceStamps argument to SingleBranchScheduler

This a continuation from pull request #739. Buildbot trac ticket 2490.

When multiple codebases are used and createAbsoluteSourceStamps is enabled, it uses the last revision seen for each codebase without changes when creating the buildset. This stops any changes from the time the buildset it created and source step is run from being included in the build.