setparents: drop copies from dropped p2 (issue3843)

httppeer: avoid large dumps when we don't see an hgweb repo When we don't get an hgweb protocol response, we dump the response to the user for diagnostic purposes (it might be a cgitb message, for instance). But if we try to clone a bundle, we don't want to show the entire bundle in the error message. Also, we don't want fetch the full bundle multiple times during fallback. So we only fetch 1k here.