Merge branch 'sg/set-ceiling-during-tests' into maint

Buggy tests could damage repositories outside the throw-away test
area we created.  We now by default export GIT_CEILING_DIRECTORIES
to limit the damage from such a stray test.

* sg/set-ceiling-during-tests:
  test-lib: set GIT_CEILING_DIRECTORIES to protect the surrounding repository

Author: gitster

t/test-lib.sh

@@ -1343,7 +1343,8 @@ fi
 GIT_TEMPLATE_DIR="$GIT_BUILD_DIR"/templates/blt
 GIT_CONFIG_NOSYSTEM=1
 GIT_ATTR_NOSYSTEM=1
-export PATH GIT_EXEC_PATH GIT_TEMPLATE_DIR GIT_CONFIG_NOSYSTEM GIT_ATTR_NOSYSTEM
+GIT_CEILING_DIRECTORIES="$TRASH_DIRECTORY/.."
+export PATH GIT_EXEC_PATH GIT_TEMPLATE_DIR GIT_CONFIG_NOSYSTEM GIT_ATTR_NOSYSTEM GIT_CEILING_DIRECTORIES
 
 if test -z "$GIT_TEST_CMP"
 then