ll-merge: make flag easier to populate

ll-merge: make flag easier to populate

ll_merge() takes its options in a flag word, which has a few
advantages:

 - options flags can be cheaply passed around in registers, while
   an option struct passed by pointer cannot;

 - callers can easily pass 0 without trouble for no options,
   while an option struct passed by value would not allow that.

The downside is that code to populate and access the flag word can be
somewhat opaque.  Mitigate that with a few macros.

Cc: Avery Pennarun <apenwarr@gmail.com>
Cc: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>