[RFC] send-email: avoid duplicate In-Reply-To and References headers

When parsing an mbox, it is possible to get existing In-Reply-To
and References headers blindly appended into the headers of
message we generate.   This is probably the wrong thing to do
and we should prioritize what was given in the command-line,
cover letter, and previously-sent messages.

One example I've noticed in the wild was:

https://public-inbox.org/git/20161111124541.8216-17-vascomalmeida@sapo.pt/raw
---
 I'm not completely sure this is what Vasco was doing in that
 message, so it's an RFC for now...

 git-send-email.perl | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/git-send-email.perl b/git-send-email.perl
index 068d60b3e6..5ab3d8585c 100755
--- a/git-send-email.perl
+++ b/git-send-email.perl

@@ -1543,7 +1543,13 @@ foreach my $t (@files) {
 			elsif (!/^Date:\s/i && /^[-A-Za-z]+:\s+\S/) {
 				push @xh, $_;
 			}
-
+			elsif (/^(?:References|In-Reply-To):/i) {
+				if (defined $initial_reply_to || $thread) {
+					warn
+"Ignoring $_ header in mbox body since it conflicts with\n
+--in-reply-to and --thread switches\n"
+				}
+			}
 		} else {
 			# In the traditional
 			# "send lots of email" format,

-- 
EW