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

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