t/perf/aggregate.perl: tolerate leading spaces

t/perf/aggregate.perl: tolerate leading spaces

When using `test_size` with `wc -c`, users on certain platforms can run
into issues when `wc` emits leading space characters in its output,
which confuses get_times.

Callers could switch to use test_file_size instead of `wc -c` (the
former never prints leading space characters, so will always work with
test_size regardless of platform), but this is an easy enough spot to
miss that we should teach get_times to be more tolerant of the input it
accepts.

Teach get_times to do just that by stripping any leading space
characters.

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