encoding: use i.startswith() instead of i[0] to eliminate py2/3 divergence

mercurial/encoding.py

@@ -29,10 +29,7 b' if pycompat.ispy3:'
            "200c 200d 200e 200f 202a 202b 202c 202d 202e "
            "206a 206b 206c 206d 206e 206f feff".split()]
 # verify the next function will work
-if pycompat.ispy3:
-    assert set(i[0] for i in _ignore) == {ord(b'\xe2'), ord(b'\xef')}
-else:
-    assert set(i[0] for i in _ignore) == {"\xe2", "\xef"}
+assert all(i.startswith(("\xe2", "\xef")) for i in _ignore)
 
 def hfsignoreclean(s):
     """Remove codepoints ignored by HFS+ from s.