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"