environment.c: enable core.preloadindex by default

environment.c: enable core.preloadindex by default

Many people are on filesystems with horrible stat latency (not
limited to Windows but also NFS), which core.preloadindex was
designed to help.  We discussed enabling it by default early in 2013
but didn't.

Per

  http://thread.gmane.org/gmane.comp.version-control.git/219273/focus=219322

let's enable the setting by default, with the original choice of max
20 threads / min 500 paths per thread parameters.

Signed-off-by: Steve Hoelzer <shoelzer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>