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.