webcommands: test that fctx is not None in filediff()

A block of code above this one already says "if fctx is not None", and it's
also what this code actually intends to check, so let's be specific as PEP-8
recommends.

mercurial/hgweb/webcommands.py

@@ -792,7 +792,7 b' def filediff(web, req, tmpl):'
         style = req.form['style'][0]
 
     diffs = webutil.diffs(web.repo, tmpl, ctx, None, [path], parity, style)
-    if fctx:
+    if fctx is not None:
         rename = webutil.renamelink(fctx)
         ctx = fctx
     else: