diff -r cf065c6a0197 -r 66af68d4c751 mercurial/pycompat.py
--- a/mercurial/pycompat.py	Fri Dec 13 10:10:40 2019 -0800
+++ b/mercurial/pycompat.py	Thu Dec 12 10:26:09 2019 -0800
@@ -253,6 +253,8 @@
         This never raises UnicodeEncodeError, but only ASCII characters
         can be round-trip by sysstr(sysbytes(s)).
         """
+        if isinstance(s, bytes):
+            return s
         return s.encode('utf-8')
 
     def sysstr(s):