Correctly set gc options in preserve-packs script

Change-Id: Ie06f69509dd882d202ecf1c5895ce8fef8db6751
diff --git a/container-images/git-gc/tools/git-gc-preserve b/container-images/git-gc/tools/git-gc-preserve
index e31a0e1..f658ae2 100755
--- a/container-images/git-gc/tools/git-gc-preserve
+++ b/container-images/git-gc/tools/git-gc-preserve
@@ -116,4 +116,9 @@
 
 prune_preserved "$repopath"
 preserve_packs "$repopath"
-git gc ${args:+"$args"}
+
+if test -n "$args"; then
+  git gc "$args"
+else
+  git gc
+fi