Merge branch 'jt/avoid-ls-refs-with-http'

Merge branch 'jt/avoid-ls-refs-with-http'

The http transport lacked some optimization the native transports
learned to avoid unnecessary ref advertisement, which has been
corrected.

* jt/avoid-ls-refs-with-http:
  transport: teach all vtables to allow fetch first
  transport-helper: skip ls-refs if unnecessary