sysroot: Add ostree_sysroot_prepare_cleanup()

New public function works like ostree_sysroot_cleanup() EXCEPT FOR
pruning the repository.

Under the hood, add _ostree_sysroot_piecemeal_cleanup() which takes
flags to better control what files are cleaned up.  Both public cleanup
functions are now wrappers for _ostree_sysroot_piecemeal_cleanup() with
different flags.

src/libostree/ostree-sysroot.h

@@ -81,6 +81,10 @@ gboolean ostree_sysroot_cleanup (OstreeSysroot       *self,
                                  GCancellable        *cancellable,
                                  GError             **error);
 
+gboolean ostree_sysroot_prepare_cleanup (OstreeSysroot  *self,
+                                         GCancellable   *cancellable,
+                                         GError        **error);
+
 gboolean ostree_sysroot_write_origin_file (OstreeSysroot         *sysroot,
                                            OstreeDeployment      *deployment,
                                            GKeyFile              *new_origin,