Fixed issue when node didn't exists at 'tip' and we tried calculate history based on that assumption. Now fallback to the changeset the node exists in for history calculation
"""Various utilities to work with Python < 2.7.Those utilities may be deleted once ``vcs`` stops support for older Pythonversions."""importsysifsys.version_info>=(2,7):unittest=__import__('unittest')else:unittest=__import__('unittest2')