rev-list: use hdr_termination instead of a always using a newline

When adding support for prefixing output of log and other commands using
--line-prefix, commit 660e113ce1 ("graph: add support for
--line-prefix on all graph-aware output", 2016-08-31) accidentally
broke rev-list --header output.

In order to make the output appear with a line-prefix, the flow was
changed to always use the graph subsystem for display. Unfortunately
the graph flow in rev-list did not use info->hdr_termination as it was
assumed that graph output would never need to putput NULs.

Since we now always use the graph code in order to handle the case of
line-prefix, simply replace putchar('\n') with
putchar(info->hdr_termination) which will correct this issue.

Add a test for the --header case to make sure we don't break it in the
future.

Reported-by: Dennis Kaarsemaker <dennis@kaarsemaker.net>
Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rev-list.c

@@ -145,7 +145,7 @@ static void show_commit(struct commit *commit, void *data)
 			 */
 			if (buf.len && buf.buf[buf.len - 1] == '\n')
 				graph_show_padding(revs->graph);
-			putchar('\n');
+			putchar(info->hdr_termination);
 		} else {
 			/*
 			 * If the message buffer is empty, just show

t/t6000-rev-list-misc.sh

@@ -100,4 +100,18 @@ test_expect_success '--bisect and --first-parent can not be combined' '
 	test_must_fail git rev-list --bisect --first-parent HEAD
 '
 
+test_expect_success '--header shows a NUL after each commit' '
+	# We know that there is no Q in the true payload; names and
+	# addresses of the authors and the committers do not have
+	# any, and object names or header names do not, either.
+	git rev-list --header --max-count=2 HEAD |
+	nul_to_q |
+	grep "^Q" >actual &&
+	cat >expect <<-EOF &&
+	Q$(git rev-parse HEAD~1)
+	Q
+	EOF
+	test_cmp expect actual
+'
+
 test_done