Merge branch 'nd/reset-setup-worktree' into maint

"git reset" needs to refresh the index when working in a working
tree (it can also be used to match the index to the HEAD in an
otherwise bare repository), but it failed to set up the working
tree properly, causing GIT_WORK_TREE to be ignored.

* nd/reset-setup-worktree:
  reset: optionally setup worktree and refresh index on --mixed