send-email: restore --in-reply-to superseding behavior

git send-email --in-reply-to= fails to override In-Reply-To email headers,
if they're present in the output of format-patch, even when explicitly
told to do so by the option --no-thread, which breaks the contract of the
command line switch option, per its man page.

"
   --in-reply-to=<identifier>
       Make the first mail (or all the mails with --no-thread) appear as
       a reply to the given Message-Id, which avoids breaking threads to
       provide a new patch series.
"

This patch fixes the aformentioned issue, by bringing --in-reply-to's old
overriding behavior back.

The test was donated by Carlo Marcelo Arenas Belón.

Signed-off-by: Rafael Aquini <aquini@redhat.com>
Helped-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -1699,10 +1699,14 @@ sub process_file {
 				$xfer_encoding = $1 if not defined $xfer_encoding;
 			}
 			elsif (/^In-Reply-To: (.*)/i) {
-				$in_reply_to = $1;
+				if (!$initial_in_reply_to || $thread) {
+					$in_reply_to = $1;
+				}
 			}
 			elsif (/^References: (.*)/i) {
-				$references = $1;
+				if (!$initial_in_reply_to || $thread) {
+					$references = $1;
+				}
 			}
 			elsif (!/^Date:\s/i && /^[-A-Za-z]+:\s+\S/) {
 				push @xh, $_;

t/t9001-send-email.sh

@@ -42,7 +42,8 @@ clean_fake_sendmail () {
 }
 
 test_expect_success $PREREQ 'Extract patches' '
-	patches=$(git format-patch -s --cc="One <one@example.com>" --cc=two@example.com -n HEAD^1)
+	patches=$(git format-patch -s --cc="One <one@example.com>" --cc=two@example.com -n HEAD^1) &&
+	threaded_patches=$(git format-patch -o threaded -s --in-reply-to="format" HEAD^1)
 '
 
 # Test no confirm early to ensure remaining tests will not hang
@@ -1219,6 +1220,17 @@ test_expect_success $PREREQ 'threading but no chain-reply-to' '
 	grep "In-Reply-To: " stdout
 '
 
+test_expect_success $PREREQ 'override in-reply-to if no threading' '
+	git send-email \
+		--dry-run \
+		--from="Example <nobody@example.com>" \
+		--to=nobody@example.com \
+		--no-thread \
+		--in-reply-to="override" \
+		$threaded_patches >stdout &&
+	grep "In-Reply-To: <override>" stdout
+'
+
 test_expect_success $PREREQ 'sendemail.to works' '
 	git config --replace-all sendemail.to "Somebody <somebody@ex.com>" &&
 	git send-email \