lib: enforce that n_message_reindex takes headers from first file

This is still a bit stopgap to be only choosing one set of headers,
but this seems like a more defensible set of headers to choose.

lib/message.cc

@@ -2034,6 +2034,8 @@ notmuch_message_reindex (notmuch_message_t *message,
 	    thread_id = orig_thread_id;
 
 	_notmuch_message_add_term (message, "thread", thread_id);
+	/* Take header values only from first filename */
+	if (found == 0)
 	    _notmuch_message_set_header_values (message, date, from, subject);
 
 	ret = _notmuch_message_index_file (message, message_file);

test/T670-duplicate-mid.sh

@@ -41,7 +41,6 @@ notmuch search --output=files "sekrit" | notmuch_dir_sanitize > OUTPUT
 test_expect_equal_file EXPECTED OUTPUT
 
 test_begin_subtest 'reindex choses subject from first filename'
-test_subtest_known_broken
 cat <<EOF > EXPECTED
 thread:XXX   2001-01-05 [1/1(3)] Notmuch Test Suite; message 0 (inbox unread)
 EOF