revset: add a 'descend' argument to followlines to return descendants This is useful to follow changes in a block of lines forward in the history (for instance, when one wants to find out how a function evolved from a point in history). We added a 'descend' parameter to followlines(), which defaults to False. If True, followlines() returns descendants of startrev. Because context.blockdescendants() does not follow renames, these are not followed by the revset either, so history will end when a rename occurs (as can be seen in tests).

context: add a blockdescendants function This is symmetrical with blockancestors() and yields descendants of a filectx with changes in the given line range. The noticeable difference is that the algorithm does not follow renames (probably because filelog.descendants() does not), so we are missing branches with renames.