send-email: don't duplicate Reply-to: in intro message

send-email: don't duplicate Reply-to: in intro message

If I run

  git send-email --compose --reply-to 'ME <my@address.net>' .....

and edit the intro message, then it will get two copies of the Reply-To
field.  gmail.com rejects such messages.

This happens because send-email reads the edited message examining the
headers.  For recognised headers the content is extracted to use in
constructing the final message and for possible inclusion in the patch
emails.  Unrecognised headers are gathered (in @xh) to be passed through
uninterpreted.

Unfortunately "Reply-To" is not recognised in this process so it is
added to @xh as an uninterpreted header, but also generated from the
$reply_to variable in gen_header(), resulting in two copies

Add parsing to the loop in pre_process_file() to recognise a Reply-to
header and to store the result in $reply_to.  This means that the
intro message will not get a second header and also means that
any changes made to the Reply-To header during editing will be
incorporated in the $reply_to variable and so included in all the
generated email messages.

Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>