t/perf: avoid redundant use of cat

Take care to redirect stdin, otherwise the output of wc would also contain
the file name.

Signed-off-by: Beat Bolli <dev+git@drbeat.li>
Acked-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/perf/repos/inflate-repo.sh

@@ -33,7 +33,7 @@ do
 done
 
 git ls-tree -r HEAD >GEN_src_list
-nr_src_files=$(cat GEN_src_list | wc -l)
+nr_src_files=$(wc -l <GEN_src_list)
 
 src_branch=$(git symbolic-ref --short HEAD)