Merge branch 'ab/test-env'

Merge branch 'ab/test-env'

Many GIT_TEST_* environment variables control various aspects of
how our tests are run, but a few followed "non-empty is true, empty
or unset is false" while others followed the usual "there are a few
ways to spell true, like yes, on, etc., and also ways to spell
false, like no, off, etc." convention.

* ab/test-env:
  env--helper: mark a file-local symbol as static
  tests: make GIT_TEST_FAIL_PREREQS a boolean
  tests: replace test_tristate with "git env--helper"
  tests README: re-flow a previously changed paragraph
  tests: make GIT_TEST_GETTEXT_POISON a boolean
  t6040 test: stop using global "script" variable
  config.c: refactor die_bad_number() to not call gettext() early
  env--helper: new undocumented builtin wrapping git_env_*()
  config tests: simplify include cycle test