webcommands: get correct parents when comparing a removed file (issue4962)

When comparing a file that was removed at the current revision, parents used to
show grandparents instead, due to how fctx was "shifted" from the current
revision to its p1. Let's not do that.

The fix is pretty much copied from webcommands.filediff().

mercurial/hgweb/webcommands.py

@@ -854,6 +854,7 b' def comparison(web, req, tmpl):'
             return [_('(binary file %s, hash: %s)') % (mt, hex(f.filenode()))]
         return f.data().splitlines()
 
+    fctx = None
     parent = ctx.p1()
     leftrev = parent.rev()
     leftnode = parent.node()
@@ -869,10 +870,14 b' def comparison(web, req, tmpl):'
             leftlines = filelines(pfctx)
     else:
         rightlines = ()
-        fctx = ctx.parents()[0][path]
-        leftlines = filelines(fctx)
+        pfctx = ctx.parents()[0][path]
+        leftlines = filelines(pfctx)
 
     comparison = webutil.compare(tmpl, context, leftlines, rightlines)
+    if fctx is not None:
+        ctx = fctx
+    else:
+        ctx = ctx
     return tmpl('filecomparison',
                 file=path,
                 node=hex(ctx.node()),
@@ -884,8 +889,8 b' def comparison(web, req, tmpl):'
                 author=ctx.user(),
                 rename=rename,
                 branch=webutil.nodebranchnodefault(ctx),
-                parent=webutil.parents(fctx),
-                child=webutil.children(fctx),
+                parent=webutil.parents(ctx),
+                child=webutil.children(ctx),
                 tags=webutil.nodetagsdict(web.repo, ctx.node()),
                 bookmarks=webutil.nodebookmarksdict(web.repo, ctx.node()),
                 leftrev=leftrev,

tests/test-hgweb-diffs.t

@@ -927,7 +927,7 b' comparison removed file'
   </tr>
   <tr>
    <th>parents</th>
-   <td><a href="/file/0cd96de13884/a">0cd96de13884</a> </td>
+   <td><a href="/file/d73db4d812ff/a">d73db4d812ff</a> </td>
   </tr>
   <tr>
    <th>children</th>