revset: make follow() reject more than one start revisions Taking only the last revision is inconsistent because ancestors(set) follows all revisions given, and theoretically follow(startrev=set) == ancestors(set). I'm planning to add a support for multiple start revisions, but that won't fit to the 4.0 time frame. So reject multiple revisions now to avoid future BC. len(revs) might be slow if revs were large, but we don't care since a valid revs should have only one element.