Merge branch 'ew/fetch-hiderefs'

A new "fetch.hideRefs" option can be used to exclude specified refs
from "rev-list --objects --stdin --not --all" traversal for
checking object connectivity, most useful when there are many
unrelated histories in a single repository.

* ew/fetch-hiderefs:
  fetch: support hideRefs to speed up connectivity checks

diff --cc builtin/fetch.c
Simple merge

diff --cc builtin/rev-list.c
Simple merge

diff --cc revision.c
Simple merge