'Find which revisions this store has in common using graphwalker.
:param graphwalker: A graphwalker object.
:return: List of SHAs that are in common'

def find_common_revisions(self, graphwalker):

    haves = []
    sha = next(graphwalker)
    while sha:
        if (sha in self):
            haves.append(sha)
            graphwalker.ack(sha)
        sha = next(graphwalker)
    return haves
