GIT_TEST_SPLIT_INDEX=<boolean> forces split-index mode on the whole
test suite. Accept any boolean values that are accepted by git-config.
-GIT_TEST_PROTOCOL_VERSION=<n>, when set, overrides the
-'protocol.version' setting to n if it is less than n.
+GIT_TEST_PROTOCOL_VERSION=<n>, when set, makes 'protocol.version'
+default to n.
GIT_TEST_FULL_IN_PACK_ARRAY=<boolean> exercises the uncommon
pack-objects code path where there are more than 1024 packs even if