[PATCH v2] send-email: restore --in-reply-to superseding behavior

[Rafael Aquini <aquini@redhat.com>]

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.

Fixes: 256be1d3f0 (send-email: avoid duplicate In-Reply-To/References, 2018-04-17)
Signed-off-by: Rafael Aquini <aquini@redhat.com>
---
v2 changelog:
* conform to the command manual page, when send-email threading is requested

 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..36c47bae1d 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 || $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, $_;
-- 
2.23.0