Add calls to git_extract_argv0_path() in programs that call git_config_*

Programs that use git_config need to find the global configuration.
When runtime prefix computation is enabled, this requires that
git_extract_argv0_path() is called early in the program's main().

This commit adds the necessary calls.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

var.c

@@ -4,6 +4,7 @@
  * Copyright (C) Eric Biederman, 2005
  */
 #include "cache.h"
+#include "exec_cmd.h"
 
 static const char var_usage[] = "git var [-l | <variable>]";
 
@@ -56,6 +57,8 @@ int main(int argc, char **argv)
 		usage(var_usage);
 	}
 
+	git_extract_argv0_path(argv[0]);
+
 	setup_git_directory_gently(&nongit);
 	val = NULL;