pycompat: prevent encoding or decoding values if not required

pycompat.py has functions strurl and bytesurl which decodes and encodes the url
passed on Python 3 respectively. In some cases, strurl gets a url which is
already str and bytesurl gets a url which is already bytes. Let's prevent
encoding or decoding the values again if not required.

Differential Revision: https://phab.mercurial-scm.org/D2472

mercurial/pycompat.py

@@ -192,11 +192,15 b' if ispy3:'
 
     def strurl(url):
         """Converts a bytes url back to str"""
-        return url.decode(u'ascii')
+        if isinstance(url, bytes):
+            return url.decode(u'ascii')
+        return url
 
     def bytesurl(url):
         """Converts a str url to bytes by encoding in ascii"""
-        return url.encode(u'ascii')
+        if isinstance(url, str):
+            return url.encode(u'ascii')
+        return url
 
     def raisewithtb(exc, tb):
         """Raise exception with the given traceback"""