send-email: Use To: headers in patch files

It's a minor annoyance when you take the painstaking time to setup To:
headers for each patch in a large series, and then go out to send the
series with git-send-email and watch git ignore the To: headers in the
patch files.

Therefore, always add To: headers from a patch file to the To: headers
for that message. Keep the prompt for the blanket To: header so as to
not break scripts (and user expectations). This means even if a patch
has a To: header, git will prompt for the To: address. Otherwise, we'll
need to introduce interface breakage to either request the header for
each patch missing a To: header or default the header to whatever To:
address is found first (be it in a patch or from user input). Both of
these options don't seem very obvious/useful.

Reported-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Tested-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -1176,6 +1176,13 @@ foreach my $t (@files) {
 					$1, $_) unless $quiet;
 				push @cc, $1;
 			}
+			elsif (/^To:\s+(.*)$/) {
+				foreach my $addr (parse_address_line($1)) {
+					printf("(mbox) Adding to: %s from line '%s'\n",
+						$addr, $_) unless $quiet;
+					push @to, sanitize_address($addr);
+				}
+			}
 			elsif (/^Cc:\s+(.*)$/) {
 				foreach my $addr (parse_address_line($1)) {
 					if (unquote_rfc2047($addr) eq $sender) {

t/t9001-send-email.sh

@@ -947,6 +947,30 @@ test_expect_success $PREREQ '--no-bcc overrides sendemail.bcc' '
 	! grep "RCPT TO:<other@ex.com>" stdout
 '
 
+test_expect_success $PREREQ 'patches To headers are used by default' '
+	patch=`git format-patch -1 --to="bodies@example.com"` &&
+	test_when_finished "rm $patch" &&
+	git send-email \
+		--dry-run \
+		--from="Example <nobody@example.com>" \
+		--smtp-server relay.example.com \
+		$patch >stdout &&
+	grep "RCPT TO:<bodies@example.com>" stdout
+'
+
+test_expect_success $PREREQ 'patches To headers are appended to' '
+	patch=`git format-patch -1 --to="bodies@example.com"` &&
+	test_when_finished "rm $patch" &&
+	git send-email \
+		--dry-run \
+		--from="Example <nobody@example.com>" \
+		--to=nobody@example.com \
+		--smtp-server relay.example.com \
+		$patch >stdout &&
+	grep "RCPT TO:<bodies@example.com>" stdout &&
+	grep "RCPT TO:<nobody@example.com>" stdout
+'
+
 test_expect_success $PREREQ 'setup expect' '
 cat >email-using-8bit <<EOF
 From fe6ecc66ece37198fe5db91fa2fc41d9f4fe5cc4 Mon Sep 17 00:00:00 2001