delta: Add --if-not-exists option

I often want to have "idempotent" systems that iterate to a known
state.  If after generating a commit, the system is interrupted, I'd
like the next run to still generate a delta.  But we don't want to
regenerate if one exists, hence this option.

Closes: #375
Approved by: jlebon

src/libostree/ostree-cmdprivate.h

@@ -27,6 +27,7 @@ G_BEGIN_DECLS
 typedef struct {
   gboolean (* ostree_generate_grub2_config) (OstreeSysroot *sysroot, int bootversion, int target_fd, GCancellable *cancellable, GError **error);
   gboolean (* ostree_static_delta_dump) (OstreeRepo *repo, const char *delta_id, GCancellable *cancellable, GError **error);
+  gboolean (* ostree_static_delta_query_exists) (OstreeRepo *repo, const char *delta_id, gboolean *out_exists, GCancellable *cancellable, GError **error);
   gboolean (* ostree_static_delta_delete) (OstreeRepo *repo, const char *delta_id, GCancellable *cancellable, GError **error);
 } OstreeCmdPrivateVTable;