checkout: add an extra checkout_overwrite mode

This is for issue projectatomic/rpm-ostree#365,
an extra option of overwrite mode is added to the checkout command
so that when there is "non-directory" file already exist
during checkout, the error will be handled.

Some tests are added for regression

Closes: #1116
Approved by: cgwalters

bash/ostree

@@ -696,6 +696,7 @@ _ostree_checkout() {
         --require-hardlinks -H
         --union
         --union-add
+        --disjoint-union
         --user-mode -U
         --whiteouts
     "