Backport 1.56 and 1.68 from trunk:

Backport 1.56 and 1.68 from trunk:

1.56:
Apply diff3.txt from SF patch http://www.python.org/sf/536241

If a str or unicode method returns the original object,
make sure that for str and unicode subclasses the original
will not be returned.

This should prevent SF bug http://www.python.org/sf/460020
from reappearing.

1.68:
Fix SF bug 599128, submitted by Inyeol Lee: .replace() would do the
wrong thing for a unicode subclass when there were zero string
replacements.  The example given in the SF bug report was only one way
to trigger this; replacing a string of length >= 2 that's not found is
another.  The code would actually write outside allocated memory if
replacement string was longer than the search string.