Merge branch 'jk/tests-cleanup'

Merge branch 'jk/tests-cleanup'

* jk/tests-cleanup:
  t0001: drop subshells just for "cd"
  t0001: drop useless subshells
  t0001: use test_must_fail
  t0001: use test_config_global
  t0001: use test_path_is_*
  t0001: make symlink reinit test more careful
  t: prefer "git config --file" to GIT_CONFIG
  t: prefer "git config --file" to GIT_CONFIG with test_must_fail
  t: stop using GIT_CONFIG to cross repo boundaries
  t: drop useless sane_unset GIT_* calls
  t/test-lib: drop redundant unset of GIT_CONFIG
  t/Makefile: stop setting GIT_CONFIG