py3: use bytestr wrapper in revsetlang.formatspec()

This backs out 1c48a8278b2f and wraps expr by bytestr() instead.

mercurial/revsetlang.py

@@ -640,14 +640,15 b' def formatspec(expr, *args):'
         m = l // 2
         return '(%s or %s)' % (listexp(s[:m], t), listexp(s[m:], t))
 
+    expr = pycompat.bytestr(expr)
     ret = ''
     pos = 0
     arg = 0
     while pos < len(expr):
-        c = expr[pos:pos + 1]
+        c = expr[pos]
         if c == '%':
             pos += 1
-            d = expr[pos:pos + 1]
+            d = expr[pos]
             if d == '%':
                 ret += d
             elif d in 'dsnbr':
@@ -656,7 +657,7 b' def formatspec(expr, *args):'
             elif d == 'l':
                 # a list of some type
                 pos += 1
-                d = expr[pos:pos + 1]
+                d = expr[pos]
                 ret += listexp(list(args[arg]), d)
                 arg += 1
             else: