t/perf: use 'test_file_size' in more places

The perf test suite prefers to use test_file_size over 'wc -c' when
inside of a test_size block. One advantage is that accidentally writign
"wc -c file" (instead of "wc -c <file") does not inadvertently break the
tests (since the former will include the filename in the output of wc).

Both of the two uses of test_size use "wc -c", but let's convert those
to the more conventional test_file_size helper instead.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/perf/p5311-pack-bitmaps-fetch.sh

@@ -39,7 +39,7 @@ test_fetch_bitmaps () {
 		'
 
 		test_size "size   $title" '
-			wc -c <tmp.pack
+			test_file_size tmp.pack
 		'
 
 		test_perf "client $title (lookup=$1)" '

t/perf/p5332-multi-pack-reuse.sh

@@ -73,7 +73,7 @@ do
 		"
 
 		test_size "clone size for $nr_packs-pack scenario ($reuse-pack reuse)" '
-			wc -c <result
+			test_file_size result
 		'
 	done
 done