lib/checkout: Rename disjoint union, change to merge identical files

It turns out that librpm automatically merges identical files between
distinct packages, and this occurs in practice with Fedora today between
`chkconfig` and `initscripts` for exmaple.

Since we added this for rpm-ostree, we basically want to do what librpm does,
let's change the semantics to do a merge.  While we're here rename
to `UNION_IDENTICAL`.

Closes: #1156
Approved by: jlebon

src/libostree/ostree-repo.h

@@ -832,13 +832,13 @@ typedef enum {
  * @OSTREE_REPO_CHECKOUT_OVERWRITE_NONE: No special options
  * @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
- * @OSTREE_REPO_CHECKOUT_OVERWRITE_DISJOINT_UNION_FILES: When layering checkouts, error out if a file would be replaced, but add new files and directories
+ * @OSTREE_REPO_CHECKOUT_OVERWRITE_UNION_IDENTICAL: Like UNION_FILES, but error if files are not identical (requires hardlink checkouts)
  */
 typedef enum {
   OSTREE_REPO_CHECKOUT_OVERWRITE_NONE = 0,
   OSTREE_REPO_CHECKOUT_OVERWRITE_UNION_FILES = 1,
   OSTREE_REPO_CHECKOUT_OVERWRITE_ADD_FILES = 2, /* Since: 2017.3 */
-  OSTREE_REPO_CHECKOUT_OVERWRITE_DISJOINT_UNION_FILES = 3 /* Since: 2017.11 */
+  OSTREE_REPO_CHECKOUT_OVERWRITE_UNION_IDENTICAL = 3, /* Since: 2017.11 */
 } OstreeRepoCheckoutOverwriteMode;
 
 _OSTREE_PUBLIC