hgweb: explictly pass basectx in webutil.diffs

There's only one case where `basectx` parameter is None (over two usages), so
it's probably not worth handling the special case as it makes code-reading
harder.

Along the way, use ctx.p1() instead of checking for ctx.parents() being empty
which should not occur.

mercurial/hgweb/webcommands.py

@@ -754,13 +754,14 b' def filediff(web, req, tmpl):'
     if fctx is not None:
         path = fctx.path()
         ctx = fctx.changectx()
+    basectx = ctx.p1()
 
     parity = paritygen(web.stripecount)
     style = web.config('web', 'style', 'paper')
     if 'style' in req.form:
         style = req.form['style'][0]
 
-    diffs = webutil.diffs(web.repo, tmpl, ctx, None, [path], parity, style)
+    diffs = webutil.diffs(web.repo, tmpl, ctx, basectx, [path], parity, style)
     if fctx is not None:
         rename = webutil.renamelink(fctx)
         ctx = fctx

mercurial/hgweb/webutil.py

@@ -442,14 +442,7 b' def diffs(repo, tmpl, ctx, basectx, file'
         m = match.always(repo.root, repo.getcwd())
 
     diffopts = patch.diffopts(repo.ui, untrusted=True)
-    if basectx is None:
-        parents = ctx.parents()
-        if parents:
-            node1 = parents[0].node()
-        else:
-            node1 = nullid
-    else:
-        node1 = basectx.node()
+    node1 = basectx.node()
     node2 = ctx.node()
 
     block = []