notes: Initialize variable to appease Sun Studio

notes: Initialize variable to appease Sun Studio

Sun Studio 12 Update 1 thinks that *t could be uninitialized,
ostensibly because it doesn't take rewrite_cmd into account in its
static analysis.

    builtin/notes.c: In function `notes_copy_from_stdin':
    builtin/notes.c:419: warning: 't' might be used uninitialized in this function

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>