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

Author: gitster

t/Makefile

@@ -36,11 +36,11 @@ test: pre-clean $(TEST_LINT)
 	$(MAKE) aggregate-results-and-cleanup
 
 prove: pre-clean $(TEST_LINT)
-	@echo "*** prove ***"; GIT_CONFIG=.git/config $(PROVE) --exec '$(SHELL_PATH_SQ)' $(GIT_PROVE_OPTS) $(T) :: $(GIT_TEST_OPTS)
+	@echo "*** prove ***"; $(PROVE) --exec '$(SHELL_PATH_SQ)' $(GIT_PROVE_OPTS) $(T) :: $(GIT_TEST_OPTS)
 	$(MAKE) clean-except-prove-cache
 
 $(T):
-	@echo "*** $@ ***"; GIT_CONFIG=.git/config '$(SHELL_PATH_SQ)' $@ $(GIT_TEST_OPTS)
+	@echo "*** $@ ***"; '$(SHELL_PATH_SQ)' $@ $(GIT_TEST_OPTS)
 
 pre-clean:
 	$(RM) -r '$(TEST_RESULTS_DIRECTORY_SQ)'