Merge branch 'bp/rename-test-env-var'

Some environment variables that control the runtime options of Git
used during tests are getting renamed for consistency.

* bp/rename-test-env-var:
  t0000: do not get self-test disrupted by environment warnings
  preload-index: update GIT_FORCE_PRELOAD_TEST support
  read-cache: update TEST_GIT_INDEX_VERSION support
  fsmonitor: update GIT_TEST_FSMONITOR support
  preload-index: use git_env_bool() not getenv() for customization
  t/README: correct spelling of "uncommon"

t/t0000-basic.sh

@@ -87,6 +87,10 @@ _run_sub_test_lib_test_common () {
 		passing metrics
 		'
 
+		# Tell the framework that we are self-testing to make sure
+		# it yields a stable result.
+		GIT_TEST_FRAMEWORK_SELFTEST=t &&
+
 		# Point to the t/test-lib.sh, which isn't in ../ as usual
 		. "\$TEST_DIRECTORY"/test-lib.sh
 		EOF