p5303: add missing &&-chains

These are in a helper function, so the usual chain-lint doesn't notice
them. This function is still not perfect, as it has some git invocations
on the left-hand-side of the pipe, but it's primary purpose is timing,
not finding bugs or correctness issues.

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

t/perf/p5303-many-packs.sh

@@ -28,11 +28,11 @@ repack_into_n () {
 			push @commits, $_ if $. % 5 == 1;
 		}
 		print reverse @commits;
-	' "$1" >pushes
+	' "$1" >pushes &&
 
 	# create base packfile
 	head -n 1 pushes |
-	git pack-objects --delta-base-offset --revs staging/pack
+	git pack-objects --delta-base-offset --revs staging/pack &&
 
 	# and then incrementals between each pair of commits
 	last= &&