hgweb: use sysstr to set attribute on diff option

Attribute identifier should be `str` not `bytes`.

mercurial/hgweb/webutil.py

@@ -230,16 +230,16 b' def difffeatureopts(req, ui, section):'
         ui, untrusted=True, section=section, whitespace=True
     )
 
-    for k in (
-        b'ignorews',
-        b'ignorewsamount',
-        b'ignorewseol',
-        b'ignoreblanklines',
+    for kb, ks in (
+        (b'ignorews', 'ignorews'),
+        (b'ignorewsamount', 'ignorewsamount'),
+        (b'ignorewseol', 'ignorewseol'),
+        (b'ignoreblanklines', 'ignoreblanklines'),
     ):
-        v = req.qsparams.get(k)
+        v = req.qsparams.get(kb)
         if v is not None:
             v = stringutil.parsebool(v)
-            setattr(diffopts, k, v if v is not None else True)
+            setattr(diffopts, ks, v if v is not None else True)
 
     return diffopts