revlog: extract first byte of revlog with a slice so it's portable

mercurial/revlog.py

@@ -1521,7 +1521,7 b' class revlog(object):'
         #
         # According to `hg perfrevlogchunks`, this is ~0.5% faster for zlib
         # compressed chunks. And this matters for changelog and manifest reads.
-        t = data[0]
+        t = data[0:1]
 
         if t == 'x':
             try: