man/prune, bash: Add --commit-only flag for ostree prune

Update the man page and the auto-complete script
to include the --commit-only flag

bash/ostree

@@ -805,6 +805,7 @@ _ostree_prune() {
         --no-prune
         --refs-only
         --static-deltas-only
+        --commit-only
     "
 
     local options_with_args="