linkrev: work around linkrev to filtered entry in 'filelog' revset This revset is used by 'hg log FILENAME'. This prevent bugs when used on a repository with hidden revisions. Instead of just discarding file revisions whose linkrevs point to filtered revisions, we put them aside and post-process them trying to find a non-filtered introduction. See inline documentation for details about how it works. This only fixes some of the problems. Once again, more will be needed when we can cannot rely on child revisions of a file to find linkrev-shadowned revisions. A test is added for 'hg log' catching such cases.