git-gui: add Japanese language code

Signed-off-by: Satoshi Yasushima <s.yasushima@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

po/ja.po

@@ -11,7 +11,7 @@ msgstr ""
 "PO-Revision-Date: 2010-02-02 19:03+0900\n"
 "Last-Translator: しらいし ななこ <nanako3@lavabit.com>\n"
 "Language-Team: Japanese\n"
-"Language: \n"
+"Language: ja\n"
 "MIME-Version: 1.0\n"
 "Content-Type: text/plain; charset=UTF-8\n"
 "Content-Transfer-Encoding: 8bit\n"