preload-index: add override to enable testing preload-index

preload-index: add override to enable testing preload-index

By default, the preload index code path doesn't run unless there is a
minimum of 1000 files. To enable running the test suite and having it
execute the preload-index path, add an environment variable
(GIT_FORCE_PRELOAD_TEST) which will override that minimum and set it to 2.

This enables you run existing tests and have the core.preloadindex code
path execute as long as the test has at least 2 files by setting
GIT_FORCE_PRELOAD_TEXT=1 before running the test.

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>