log: fix --follow FILE ancestry calculation Currently, --follow FILE looks for a FILE filelog, scans it and collects linkrevs and renames, then filters them. The problem is the filelog scan does not start at FILE filenode in parent revision but at the last filelog revision. So: - Files not in the parent revision can be followed, the starting node is unexpected - Files in the parent revision can be followed from an incorrect starting node. This patch makes log --follow FILE fail if FILE is not in parent revision, and computes ancestors of the parent revision FILE filenode.