commit_tree(): refuse commit messages that contain NULs

Current implementation sees NUL as terminator. If users give a message
with NUL byte in it (e.g. editor set to save as UTF-16), the new commit
message will have NULs. However following operations (displaying or
amending a commit for example) will not keep anything after the first NUL.

Stop user right when they do this. If NUL is added by mistake, they have
their chance to fix. Otherwise, log messages will no longer be text "git
log" and friends would grok.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

diff --git a/commit.c b/commit.c
index 0a214a6..80e61b4 100644
--- a/commit.c
+++ b/commit.c
@@ -855,6 +855,9 @@
 
 	assert_sha1_type(tree, OBJ_TREE);
 
+	if (memchr(msg->buf, '\0', msg->len))
+		return error("a NUL byte in commit log message not allowed.");
+
 	/* Not having i18n.commitencoding is the same as having utf-8 */
 	encoding_is_utf8 = is_encoding_utf8(git_commit_encoding);
 

t/t3900-i18n-commit.sh

diff --git a/t/t3900-i18n-commit.sh b/t/t3900-i18n-commit.sh
index 1f62c15..d48a7c0 100755
--- a/t/t3900-i18n-commit.sh
+++ b/t/t3900-i18n-commit.sh
@@ -34,6 +34,12 @@
 	test z = "z$E"
 '
 
+test_expect_failure 'UTF-16 refused because of NULs' '
+	echo UTF-16 >F &&
+	git commit -a -F "$TEST_DIRECTORY"/t3900/UTF-16.txt
+'
+
+
 for H in ISO8859-1 eucJP ISO-2022-JP
 do
 	test_expect_success "$H setup" '