[PATCH] send-email: restore --in-reply-to superseding behavior

[Rafael Aquini <aquini@redhat.com>]

git send-email --in-reply-to= fails to override the email headers,
if they're present in the output of format-patch, which breakes the
contract of the command line switch. This can cause an email thread to
break, if subsequent replies to a given message need to be sent after
amending fixes to a commit and re-running git format-patch to get the
incremental patch-fix posted.

Fixes: 256be1d3f0 (send-email: avoid duplicate In-Reply-To/References, 2018-04-17)
Signed-off-by: Rafael Aquini <aquini@redhat.com>
---
 git-send-email.perl | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/git-send-email.perl b/git-send-email.perl
index dc95656f75..342e00b1f3 100755
--- a/git-send-email.perl
+++ b/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) {
+					$in_reply_to = $1;
+				}
 			}
 			elsif (/^References: (.*)/i) {
-				$references = $1;
+				if (!$initial_in_reply_to) {
+					$references = $1;
+				}
 			}
 			elsif (!/^Date:\s/i && /^[-A-Za-z]+:\s+\S/) {
 				push @xh, $_;
-- 
2.23.0