t6501: use 'git gc' in quiet mode

t6501: use 'git gc' in quiet mode

t6501-freshen-objects.sh sends the standard error from
'git gc' to a file and verifies that it is empty. This
is intended as a way to ensure no warnings are written
during the operation. However, as the commit-graph is
added as a step to 'git gc', its progress will appear
in the output.

Pass the '-q' argument to avoid a failing test case
when progress is written.

Signed-off-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>