ll-merge: replace flag argument with options struct

ll-merge: replace flag argument with options struct

Keeping track of the flag bits is proving more trouble than it's
worth.  Instead, use a pointer to an options struct like most similar
APIs do.

Callers with no special requests can pass NULL to request the default
options.

Cc: Bert Wesarg <redacted>
Cc: Avery Pennarun <redacted>
Helped-by: Justin Frankel <redacted>
Helped-by: Bert Wesarg <redacted>
Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>