hgweb: return content iterator instead of using write() callable
This is a new version of
4879468fa28f (which was backed out in
943f066c0d58),
with an extra line removed to fix problems with hg serve. hg's internal web
server contains checking if the app isn't trying to write more bytes than
specified by the Content-Length header. The first try still contained an old
line that wrote the response, so the response was sent twice.
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