hgweb: make "comparison" get parent from not filelog but changelog Before this patch, "comparison" shows unexpected result, when the specified file is not modified in the specified revision, even though "diff" shows empty result. When REV doesn't change specified FILE, "diff" shows: "hg diff -c REV FILE" but "comparison" shows: "hg diff -c `hg parents -r REV FILE` FILE" In other words, the former gets parent from changelog, but the latter gets one from filelog. This may confuse users browsing (and switching "diff" and "comparison" of) files in the specified revision. This patch makes "comparison" get parent from not filelog but changelog, to show "hg diff -c REV FILE" in both "diff" and "comparison" pages.