[PATCH] py3: make py3 compat.iterbytestr simpler and faster

[Martin von Zweigbergk]

# HG changeset patch
# User Martin von Zweigbergk <martinvonz at google.com>
# Date 1489528461 25200
#      Tue Mar 14 14:54:21 2017 -0700
# Node ID 375345ceabc9f14e780d8a8efd00ed32ffc6396c
# Parent  3d3109339b57341b333c1112beb41dd281fa944a
py3: make py3 compat.iterbytestr simpler and faster

I don't have a py3.5 installed, so I have not run tests on this, but
maybe it works?

$ python3 -m timeit -s 's=b"a"*100' 'for x in iter(s[i:i + 1] for i in range(len(s))): pass'
100000 loops, best of 3: 10.3 usec per loop

$ python3 -m timeit -s 's=b"a"*100' 'for x in map(chr, s): pass'
100000 loops, best of 3: 6.16 usec per loop

(I picked the best "before" time and worst "after" time out of a few runs.)

diff -r 3d3109339b57 -r 375345ceabc9 mercurial/pycompat.py
--- a/mercurial/pycompat.py	Mon Mar 13 11:19:24 2017 -0700
+++ b/mercurial/pycompat.py	Tue Mar 14 14:54:21 2017 -0700
@@ -78,7 +78,7 @@
 
     def iterbytestr(s):
         """Iterate bytes as if it were a str object of Python 2"""
-        return iter(s[i:i + 1] for i in range(len(s)))
+        return map(chr, s)
 
     def sysstr(s):
         """Return a keyword str to be passed to Python functions such as