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

builtin/notes.c

@@ -416,7 +416,7 @@ int notes_copy_from_stdin(int force, const char *rewrite_cmd)
 {
 	struct strbuf buf = STRBUF_INIT;
 	struct notes_rewrite_cfg *c = NULL;
-	struct notes_tree *t;
+	struct notes_tree *t = NULL;
 	int ret = 0;
 
 	if (rewrite_cmd) {