py3: add b'' prefixes in tests/test-custom-filters.t

# skip-blame because just b'' prefixes

Differential Revision: https://phab.mercurial-scm.org/D3595

tests/test-custom-filters.t

@@ -12,15 +12,15 b''
   $ cat > prefix.py <<EOF
   > from mercurial import error
   > def stripprefix(s, cmd, filename, **kwargs):
-  >     header = '%s\n' % cmd
+  >     header = b'%s\n' % cmd
   >     if s[:len(header)] != header:
-  >         raise error.Abort('missing header "%s" in %s' % (cmd, filename))
+  >         raise error.Abort(b'missing header "%s" in %s' % (cmd, filename))
   >     return s[len(header):]
   > def insertprefix(s, cmd):
-  >     return '%s\n%s' % (cmd, s)
+  >     return b'%s\n%s' % (cmd, s)
   > def reposetup(ui, repo):
-  >     repo.adddatafilter('stripprefix:', stripprefix)
-  >     repo.adddatafilter('insertprefix:', insertprefix)
+  >     repo.adddatafilter(b'stripprefix:', stripprefix)
+  >     repo.adddatafilter(b'insertprefix:', insertprefix)
   > EOF
 
   $ cat > .hgignore <<EOF