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 <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: derrickstolee

t/t6501-freshen-objects.sh

@@ -137,7 +137,7 @@ test_expect_success 'do not complain about existing broken links (commit)' '
 	some message
 	EOF
 	commit=$(git hash-object -t commit -w broken-commit) &&
-	git gc 2>stderr &&
+	git gc -q 2>stderr &&
 	verbose git cat-file -e $commit &&
 	test_must_be_empty stderr
 '
@@ -147,7 +147,7 @@ test_expect_success 'do not complain about existing broken links (tree)' '
 	100644 blob 0000000000000000000000000000000000000003	foo
 	EOF
 	tree=$(git mktree --missing <broken-tree) &&
-	git gc 2>stderr &&
+	git gc -q 2>stderr &&
 	git cat-file -e $tree &&
 	test_must_be_empty stderr
 '
@@ -162,7 +162,7 @@ test_expect_success 'do not complain about existing broken links (tag)' '
 	this is a broken tag
 	EOF
 	tag=$(git hash-object -t tag -w broken-tag) &&
-	git gc 2>stderr &&
+	git gc -q 2>stderr &&
 	git cat-file -e $tag &&
 	test_must_be_empty stderr
 '