hgweb: fixes invalid parents / children in comparison

Previously, the parents / children were computed relative to the cset of the
currently shown file, which was wrong and inconsistent with diff and others.
With this patch, the listed csets are those that contain changes to the
currently compared file, which don't necessarily have to be the direct parents
and children of the changeset itself.

mercurial/hgweb/webcommands.py

@@ -639,8 +639,8 b' def comparison(web, req, tmpl):'
                 author=ctx.user(),
                 rename=rename,
                 branch=webutil.nodebranchnodefault(ctx),
-                parent=webutil.parents(ctx),
-                child=webutil.children(ctx),
+                parent=webutil.parents(fctx),
+                child=webutil.children(fctx),
                 leftrev=leftrev,
                 leftnode=hex(leftnode),
                 rightrev=rightrev,

tests/test-hgweb-diffs.t

@@ -628,7 +628,7 b' comparison new file'
   </tr>
   <tr>
    <th>children</th>
-   <td><a href="/file/559edbd9ed20/a">559edbd9ed20</a> </td>
+   <td></td>
   </tr>
   
   </table>
@@ -746,7 +746,7 b' comparison existing file'
   </tr>
   <tr>
    <th>parents</th>
-   <td><a href="/file/559edbd9ed20/a">559edbd9ed20</a> </td>
+   <td><a href="/file/0cd96de13884/a">0cd96de13884</a> </td>
   </tr>
   <tr>
    <th>children</th>
@@ -870,7 +870,7 b' comparison removed file'
   </tr>
   <tr>
    <th>parents</th>
-   <td><a href="/file/d73db4d812ff/a">d73db4d812ff</a> </td>
+   <td><a href="/file/0cd96de13884/a">0cd96de13884</a> </td>
   </tr>
   <tr>
    <th>children</th>