context: start walking from "introrev" in blockancestors() Previously, calling blockancestors() with a fctx not touching file would sometimes yield this filectx first, instead of the first "block ancestor", because when compared to its parent it may have changes in specified line range despite not touching the file at all. Fixing this by starting the algorithm from the "base" filectx obtained using fctx.introrev() (as done in annotate()). In tests, add a changeset not touching file we want to follow lines of to cover this case. Do this in test-annotate.t for followlines revset tests and in test-hgweb-filelog.t for /log/<rev>/<file>?linerange=<from>:<to> tests.