WIP: Adapt to and use new ostree-ext tar-split branch

Depends ostreedev/ostree-rs-ext#123

We gain a new `rpm-ostree container-encapsulate` option which
is like `ostree container encapsulate`, but generates chunked
images using the RPM database.

And on the client side, we now know how to handle incremental
updates - chunks that haven't changed won't be redownloaded.

Cargo.toml

@@ -109,3 +109,7 @@ bin-unit-tests = []
 sanitizers = []
 
 default = []
+
+[patch.crates-io]
+ostree-ext = { git = "https://github.com/cgwalters/ostree-rs-ext", branch = "tar-split" }
+#ostree-ext = { path = "../../ostreedev/ostree-rs-ext/lib" }

rust/src/composepost.rs

@@ -43,7 +43,7 @@ pub(crate) static COMPAT_VARLIB_SYMLINKS: &[&str] = &["alternatives", "vagrant"]
 
 /* See rpmostree-core.h */
 const RPMOSTREE_BASE_RPMDB: &str = "usr/lib/sysimage/rpm-ostree-base-db";
-const RPMOSTREE_RPMDB_LOCATION: &str = "usr/share/rpm";
+pub(crate) const RPMOSTREE_RPMDB_LOCATION: &str = "usr/share/rpm";
 const RPMOSTREE_SYSIMAGE_RPMDB: &str = "usr/lib/sysimage/rpm";
 pub(crate) const TRADITIONAL_RPMDB_LOCATION: &str = "var/lib/rpm";