t0021/rot13-filter: refactor checking final lf

As checking for a lf character at the end of a buffer
will be useful in another function, let's refactor this
functionality into a small remove_final_lf_or_die()
helper function.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: chriscool

t/t0021/rot13-filter.pl

@@ -93,12 +93,20 @@ sub packet_bin_read {
 	}
 }
 
-sub packet_txt_read {
-	my ( $res, $buf ) = packet_bin_read();
-	unless ( $res == -1 or $buf eq '' or $buf =~ s/\n$// ) {
+sub remove_final_lf_or_die {
+	my $buf = shift;
+	unless ( $buf =~ s/\n$// ) {
 		die "A non-binary line MUST be terminated by an LF.\n"
 		    . "Received: '$buf'";
 	}
+	return $buf;
+}
+
+sub packet_txt_read {
+	my ( $res, $buf ) = packet_bin_read();
+	unless ( $res == -1 or $buf eq '' ) {
+		$buf = remove_final_lf_or_die($buf);
+	}
 	return ( $res, $buf );
 }