t0021: compute file size with a single process instead of a pipeline

Avoid unwanted coding patterns (prodigal use of pipelines), and in
particular a useless use of cat.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Jeff King <peff@peff.net>

t/t0021-conversion.sh

@@ -22,7 +22,7 @@ generate_random_characters () {
 }
 
 file_size () {
-	cat "$1" | wc -c | sed "s/^[ ]*//"
+	perl -e 'print -s $ARGV[0]' "$1"
 }
 
 filter_git () {