t3900: ISO-2022-JP has more than one popular variants

t3900: ISO-2022-JP has more than one popular variants

When converting from other encodings (e.g. EUC-JP or UTF-8), there are
subtly different variants of ISO-2022-JP, all of which are valid.  At the
end of line or when a run of string switches to 1-byte sequence, ESC ( B
can be used to switch to ASCII or ESC ( J can be used to switch to ISO
646:JP (JIS X 0201) but they essentially are the same character set and
are used interchangeably.  Similarly the set ESC $ @ switches to (JIS X
0208-1978) and ESC $ B switches to (JIS X 0208-1983) are in practice used
interchangeably.

Depending on the iconv library and the locale definition on the system, a
program that converts from another encoding to ISO-2022-JP can produce
different byte sequence, and GIT_TEST_CMP (aka "diff -u") will report the
difference as a failure.

Fix this by converting the expected and the actual output to UTF-8 before
comparing when the end result is ISO-2022-JP.  The test vector string in
t3900/ISO-2022-JP.txt is expressed with ASCII and JIS X 0208-1983, but it
can be expressed with any other possible variant, and when converted back
to UTF-8, these variants produce identical byte sequences.

Signed-off-by: Junio C Hamano <gitster@pobox.com>