posix: use slicing to grab a single byte out of a bytes in HFS+ normcase code

mercurial/posix.py

@@ -372,7 +372,7 b" if pycompat.sysplatform == 'darwin':"
                     c = encoding.getutf8char(path, pos)
                     pos += len(c)
                 except ValueError:
-                    c = '%%%02X' % ord(path[pos])
+                    c = '%%%02X' % ord(path[pos:pos + 1])
                     pos += 1
                 s += c