repo: Create uncompressed-object-cache dir dynamically

Having the `uncompressed-object-cache` directory in `archive` repos by default
is clutter; the functionality should be considered deprecated.

Now we only create the directory if we're doing a checkout with the cache
enabled.

Closes: #1446
Approved by: jlebon

tests/basic-test.sh

@@ -42,6 +42,9 @@ else
     fi
 fi
 
+# This should be dynamic now
+assert_not_has_dir repo/uncompressed-objects-cache
+
 validate_checkout_basic() {
     (cd $1;
      assert_has_file firstfile