lib: Add a private helper to abort txns, use in sysroot cleanup

Steal some code from flatpak for this, which allows porting a few more things to
new style. I started on a public API version of this but was trying to roll some
other things into it and it snowballed. Let's do this version since it's easy
for now.

While here I changed things so that `generate_deployment_refs()` now just uses
`_set_ref_immediate()` rather than requring a txn.

Also, AFAICS there was no test coverage of `generate_deployment_refs()`; I tried
commenting it out and at least `admin-test.sh` passed. Add some coverage of this
- I verified that with this commenting out bits of that function cause the test
to fail.

Closes: #1132
Approved by: jlebon

src/libostree/ostree-repo-private.h

@@ -158,6 +158,27 @@ struct OstreeRepo {
   OstreeRepo *parent_repo;
 };
 
+/* Taken from flatpak; may be made into public API later */
+typedef OstreeRepo _OstreeRepoAutoTransaction;
+static inline void
+_ostree_repo_auto_transaction_cleanup (void *p)
+{
+  OstreeRepo *repo = p;
+  if (repo)
+    (void) ostree_repo_abort_transaction (repo, NULL, NULL);
+}
+
+static inline _OstreeRepoAutoTransaction *
+_ostree_repo_auto_transaction_start (OstreeRepo     *repo,
+                                     GCancellable   *cancellable,
+                                     GError        **error)
+{
+  if (!ostree_repo_prepare_transaction (repo, NULL, cancellable, error))
+    return NULL;
+  return (_OstreeRepoAutoTransaction *)repo;
+}
+G_DEFINE_AUTOPTR_CLEANUP_FUNC (_OstreeRepoAutoTransaction, _ostree_repo_auto_transaction_cleanup)
+
 typedef struct {
   dev_t dev;
   ino_t ino;