send-email: default to quoted-printable when CR is present

In 7a36987fff ("send-email: add an auto option for transfer encoding",
2018-07-08), git send-email learned how to automatically determine the
transfer encoding for a patch. However, the only criterion considered
was the length of the lines.

Another case we need to consider is that of carriage returns. Because
emails have CRLF endings when canonicalized, we don't want to write raw
carriage returns into a patch, lest they be stripped off as an artifact
of the transport. Ensure that we choose quoted-printable encoding if the
patch we're sending contains carriage returns.

Note that we are guaranteed to always correctly encode carriage returns
when writing quoted-printable since we explicitly specify the line
ending as "\n", forcing MIME::QuotedPrint to encode our carriage return
as "=0D".

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -1866,7 +1866,7 @@ sub apply_transfer_encoding {
 	$message = MIME::Base64::decode($message)
 		if ($from eq 'base64');
 
-	$to = ($message =~ /.{999,}/) ? 'quoted-printable' : '8bit'
+	$to = ($message =~ /(?:.{999,}|\r)/) ? 'quoted-printable' : '8bit'
 		if $to eq 'auto';
 
 	die __("cannot send message as 7bit")

t/t9001-send-email.sh

@@ -481,6 +481,20 @@ test_expect_success $PREREQ 'long lines with auto encoding are quoted-printable'
 	grep "Content-Transfer-Encoding: quoted-printable" msgtxt1
 '
 
+test_expect_success $PREREQ 'carriage returns with auto encoding are quoted-printable' '
+	clean_fake_sendmail &&
+	cp $patches cr.patch &&
+	printf "this is a line\r\n" >>cr.patch &&
+	git send-email \
+		--from="Example <nobody@example.com>" \
+		--to=nobody@example.com \
+		--smtp-server="$(pwd)/fake.sendmail" \
+		--transfer-encoding=auto \
+		--no-validate \
+		cr.patch &&
+	grep "Content-Transfer-Encoding: quoted-printable" msgtxt1
+'
+
 for enc in auto quoted-printable base64
 do
 	test_expect_success $PREREQ "--validate passes with encoding $enc" '