Merge branch 'jk/proto-v2-ref-prefix-fix'

"git fetch" over protocol v2 sent an incorrect ref prefix request to the server and made "git pull" with configured fetch refspec that does not cover the remote branch to merge with fail, which has been corrected. * jk/proto-v2-ref-prefix-fix: fetch: add branch.*.merge to default ref-prefix extension fetch: stop checking for NULL transport->remote in do_fetch()