t/perf: don't depend on Git.pm

t/perf: don't depend on Git.pm

The perf suite's aggregate.perl depends on Git.pm, which is a mild
annoyance if you've built git with NO_PERL. It turns out that the only
thing we use it for is a single call of the command_oneline() helper.
We can just replace this with backticks or similar.

Annoyingly, perl has no backtick equivalent that avoids a shell eval,
which means our $arg would require quoting. This probably doesn't matter
for our purposes, but it's better to be safe and model good style. So
we'll just provide a short helper around open(), which takes its
arguments as a list.

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