p5303: avoid sed GNU-ism

Using "1~5" isn't portable. Nobody seems to have noticed, since perhaps
people don't tend to run the perf suite on more exotic platforms. Still,
it's better to set a good example.

We can use:

  perl -ne 'print if $. % 5 == 1'

instead. But we can further observe that perl does a good job of the
other parts of this pipeline, and fold the whole thing together.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/perf/p5303-many-packs.sh

@@ -21,10 +21,14 @@ repack_into_n () {
 	mkdir staging &&
 
 	git rev-list --first-parent HEAD |
-	sed -n '1~5p' |
-	head -n "$1" |
-	perl -e 'print reverse <>' \
-	>pushes
+	perl -e '
+		my $n = shift;
+		while (<>) {
+			last unless @commits < $n;
+			push @commits, $_ if $. % 5 == 1;
+		}
+		print reverse @commits;
+	' "$1" >pushes
 
 	# create base packfile
 	head -n 1 pushes |