revlog: don't use slicing to return parents This is the only place we use a slice on index entries, which are currently tuples. In preparation for moving away from tuples, let's stop using slices so we don't have to implement that support on the new type. We also tweak the logic slightly so the exception only catches the IndexError on the index lookup, not on the index entry lookup. The old code should never have been buggy. But it was semantically wrong. Differential Revision: https://phab.mercurial-scm.org/D1764