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>

Documentation/config.txt

@@ -613,9 +613,9 @@ core.preloadindex::
 +
 This can speed up operations like 'git diff' and 'git status' especially
 on filesystems like NFS that have weak caching semantics and thus
-relatively high IO latencies.  With this set to 'true', Git will do the
+relatively high IO latencies.  When enabled, Git will do the
 index comparison to the filesystem data in parallel, allowing
-overlapping IO's.
+overlapping IO's.  Defaults to true.
 
 core.createObject::
 	You can set this to 'link', in which case a hardlink followed by

environment.c

@@ -71,7 +71,7 @@ unsigned long pack_size_limit_cfg;
 char comment_line_char = '#';
 
 /* Parallel index stat data preload? */
-int core_preload_index = 0;
+int core_preload_index = 1;
 
 /* This is set by setup_git_dir_gently() and/or git_default_config() */
 char *git_work_tree_cfg;