templater: make a template a string-only iterator

streamclone yields chunks instead of accepting a file-like object

hgweb: protocol functions take repo instead of web This makes it much easier for implementers of hgweb alternatives to simply call into protocol functions after setting up a repo and a request.

hgweb: move capabilities calculation back into hgweb.protocol

hgweb: centralize permission checks for protocol commands Consistently enforces authorization checks set up in hgrc up front, so that the actual commands don't have to worry about them and implementers of hgweb alternatives can easily implement their own permission checks.

tests: add some tests for web.allowpull configurations