checkout: no auto-detach if the ref is already checked out

Give the user a choice in this case: detach, make a new branch,
release current branch in the other checkout or simply 'cd' there and
continue to work.
checkout --to: fix dangling pointers in remove_junk()

junk_git_dir is set to sb_repo.buf. By the end of prepare_linked_checkout(),
sb_repo is freed and so junk_git_dir points to nowhere. If the second
checkout command fails, is_junk remains non-zero, remove_junk() will
be called and try to clean junk_git_dir, which could be anything now
(if it does not crash the program).

The new test may pass even without this patch. But it does fail under
valgrind (without this patch) with "Invalid read of size 8" at the
right line.
environment.c: fix incorrect git_graft_file initialization

"info/grafts" should be part of the "common repository" when accessed
from a linked checkout (iow $GIT_COMMON_DIR/info/grafts, not
$GIT_DIR/info/grafts).

git_path("info/grafts") returns correctly, even without this fix,
because it detects that $GIT_GRAFT_FILE is not set, so it goes with the
common rule: anything except sparse-checkout in 'info' belongs to common
repo. But get_graft_file() would return a wrong value and that one is
used for setting grafts up.
checkout: prefix --to argument properly when cwd is moved
checkout --to: do not touch existing target directory