checkout: Support a "pure addition" mode

I plan to use this for `rpm-ostree livefs`.
https://github.com/projectatomic/rpm-ostree/issues/639

Closes: #714
Approved by: jlebon

src/libostree/ostree-repo.h

@@ -722,11 +722,13 @@ typedef enum {
 /**
  * OstreeRepoCheckoutOverwriteMode:
  * @OSTREE_REPO_CHECKOUT_OVERWRITE_NONE: No special options
- * @OSTREE_REPO_CHECKOUT_OVERWRITE_UNION_FILES: When layering checkouts, overwrite earlier files, but keep earlier directories
+ * @OSTREE_REPO_CHECKOUT_OVERWRITE_UNION_FILES: When layering checkouts, unlink() and replace existing files, but do not modify existing directories
+ * @OSTREE_REPO_CHECKOUT_OVERWRITE_ADD_FILES: Only add new files/directories
  */
 typedef enum {
   OSTREE_REPO_CHECKOUT_OVERWRITE_NONE = 0,
-  OSTREE_REPO_CHECKOUT_OVERWRITE_UNION_FILES = 1
+  OSTREE_REPO_CHECKOUT_OVERWRITE_UNION_FILES = 1,
+  OSTREE_REPO_CHECKOUT_OVERWRITE_ADD_FILES = 2, /* Since: 2017.3 */
 } OstreeRepoCheckoutOverwriteMode;
 
 _OSTREE_PUBLIC