scm: Ruby 1.9 compatibility for diff, cat and blame (#2664).
"\xC2\x80" of latin1(iso-8859-1) is valid sequence of UTF-8. to_utf8() should not return string if it is UTF-8 valid_encoding.
Authored by: tmaruyama 2011-02-22
Parent: [r4921]
Child: [r4923]
