Add support for a "GIT_INDEX_FILE" environment variable.

We use that to specify alternative index files, which can be useful
if you want to (for example) generate a temporary index file to do
some specific operation that you don't want to mess with your main
one with.

It defaults to the regular ".git/index" if it hasn't been specified.

show-files.c

@@ -6,7 +6,6 @@
  * Copyright (C) Linus Torvalds, 2005
  */
 #include <dirent.h>
-#include <sys/param.h>
 
 #include "cache.h"