* ei/worktree+filter:
  filter-branch: always export GIT_DIR if it is set
  setup_git_directory: fix segfault if repository is found in cwd
  test GIT_WORK_TREE
  extend rev-parse test for --is-inside-work-tree
  Use new semantics of is_bare/inside_git_dir/inside_work_tree
  introduce GIT_WORK_TREE to specify the work tree
  test git rev-parse
  rev-parse: introduce --is-bare-repository
  rev-parse: document --is-inside-git-dir