status: fix null termination with "-b"

When the "-z" option is given to status, we are supposed to
NUL-terminate each record. However, the "-b" code to show
the tracking branch did not respect this, and always ended
with a newline.

Signed-off-by: Jeff King <peff@peff.net>
diff --git a/t/t7508-status.sh b/t/t7508-status.sh
index fc57b13..24728fa 100755
--- a/t/t7508-status.sh
+++ b/t/t7508-status.sh
@@ -271,6 +271,15 @@
 
 '
 
+test_expect_success 'status -s -z -b' '
+	tr "\\n" Q <expect >expect.q &&
+	mv expect.q expect &&
+	git status -s -z -b >output &&
+	nul_to_q <output >output.q &&
+	mv output.q output &&
+	test_cmp expect output
+'
+
 test_expect_success 'setup dir3' '
 	mkdir dir3 &&
 	: >dir3/untracked1 &&