Merge branch 'nd/multiple-work-trees-1' into jch
Continue polishing nd/multiple-work-trees topic in 'next'.
* nd/multiple-work-trees-1:
checkout --to: do not touch existing target directory
checkout: prefix --to argument properly when cwd is moved
environment.c: fix incorrect git_graft_file initialization
checkout --to: fix dangling pointers in remove_junk()
checkout: no auto-detach if the ref is already checked out
prune --repos: fix uninitialized access
checkout: no need to call check_linked_checkouts if head_ref is NULL
gitrepository-layout.txt: s/ignored/ignored if/