unpack-objects: read configuration data upon startup.

With this, unpack-objects will write out the loose objects with
new-style headers when core.legacyheaders configuration is set
to false.

One unfortunate thing is that we still need inflate/deflate cycle
when unpacking, even for objects in the pack stream that are not
deltified, because it is not possible to determine the boundary of
objects in the encoded stream cheaply without inflating it first.

Signed-off-by: Junio C Hamano <junkio@cox.net>

unpack-objects.c

@@ -266,6 +266,7 @@ int main(int argc, char **argv)
 	unsigned char sha1[20];
 
 	setup_git_directory();
+	git_config(git_default_config);
 
 	quiet = !isatty(2);