Update A Previous Revision Of A Snippet

Identical to `UPDATE /snippets/encoded_id`, except that this endpointtakes an explicit commit revision. Only the snippet's "HEAD"/"tip"(most recent) version can be updated and requests on all other,older revisions fail by returning a 405 status.Usage of this endpoint over the unrestricted `/snippets/encoded_id`could be desired if the caller wants to be sure no concurrentmodifications have taken place between the moment of the UPDATErequest and the original GET....