lib/sysroot: Add non-failable ostree_sysroot_repo()

Having a failable accessor is annoying, since it's really common
to reference both.  Instead, open the repo once when we load
the sysroot, and provide a non-failable accessor.

This is also prep for `ostree_repo_open_at()`, which collapses the separation
between `ostree_repo_new()` and `ostree_repo_open()`.

Closes: #886
Approved by: jlebon

src/libostree/ostree-sysroot.h

@@ -126,6 +126,9 @@ gboolean ostree_sysroot_write_origin_file (OstreeSysroot         *sysroot,
                                            GCancellable          *cancellable,
                                            GError               **error);
 
+_OSTREE_PUBLIC
+OstreeRepo * ostree_sysroot_repo (OstreeSysroot *self);
+
 _OSTREE_PUBLIC
 gboolean ostree_sysroot_get_repo (OstreeSysroot         *self,
                                   OstreeRepo           **out_repo,