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.

read-cache.c

@@ -188,7 +188,7 @@ int read_cache(void)
 		sha1_file_directory = DEFAULT_DB_ENVIRONMENT;
 	if (access(sha1_file_directory, X_OK) < 0)
 		return error("no access to SHA1 file directory");
-	fd = open(".git/index", O_RDONLY);
+	fd = open(get_index_file(), O_RDONLY);
 	if (fd < 0)
 		return (errno == ENOENT) ? 0 : error("open failed");