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"