dagop: yield intro filectx by filectxancestors() This is the convention of dagop.*ancestors() functions.

filectx: extract helper method to obtain filectx pointing to its introrev

dagop: copy basefilectx.ancestors() to free function The primary goal of this series is to make follow() support multiple start revisions. dagop.filectxancestors() will be extended to take multiple filectxs. basefilectx.ancestors() is not forwarded to this function because doing that would resurrect the performance issue fixed by 24b57c3899f8.