Move ref writing to be transaction-based

Rather than having separate write_ref calls, make clients start a
transaction, add some refs, and then commit it. While this doesn't
make it 100% atomic, it makes it easier for us to use an atomic
model, and it means we don't do as much I/O updating the summary
file and such.

https://bugzilla.gnome.org/show_bug.cgi?id=707644

doc/ostree-sections.txt

@@ -68,6 +68,8 @@ ostree_repo_scan_hardlinks
 ostree_repo_prepare_transaction
 ostree_repo_commit_transaction
 ostree_repo_abort_transaction
+ostree_repo_transaction_set_ref
+ostree_repo_transaction_set_refspec
 ostree_repo_has_object
 ostree_repo_write_metadata
 ostree_repo_write_metadata_async
@@ -78,8 +80,6 @@ ostree_repo_write_content_trusted
 ostree_repo_write_content_async
 ostree_repo_write_content_finish
 ostree_repo_resolve_rev
-ostree_repo_write_ref
-ostree_repo_write_refspec
 ostree_repo_list_refs
 ostree_repo_load_variant
 ostree_repo_load_variant_if_exists