Fix capability negotiation when fetching packs over HTTP.
Fix run_git_or_fail to let stderr be overridden.

GitHTTPServer relies on stderr being sent to nowhere, but
run_git_or_fail always sent it to stdout, causing non-side-band clients
to fail.
Merge branch 'http-fetch-capa' of git://github.com/wgrant/dulwich

Conflicts:
	NEWS