Update the "test installed Git" mode of our test suite to work better.

* js/test-git-installed:
  tests: explicitly use `git.exe` on Windows
  tests: do not require Git to be built when testing an installed Git
  t/lib-gettext: test installed git-sh-i18n if GIT_TEST_INSTALLED is set
  tests: respect GIT_TEST_INSTALLED when initializing repositories
  tests: fix GIT_TEST_INSTALLED's PATH to include t/helper/