repo: Add a private helper to replace a file, honoring fsync policy

Extracted from discussion in https://github.com/GNOME/ostree/pull/83

src/libostree/ostree-repo-private.h

@@ -126,6 +126,15 @@ _ostree_repo_update_refs (OstreeRepo        *self,
                           GCancellable      *cancellable,
                           GError           **error);
 
+gboolean      
+_ostree_repo_file_replace_contents (OstreeRepo    *self,
+                                    int            dfd,
+                                    const char    *path,
+                                    guint8        *buf,
+                                    gsize          len,
+                                    GCancellable  *cancellable,
+                                    GError       **error);
+
 gboolean      
 _ostree_repo_write_ref (OstreeRepo    *self,
                         const char    *remote,