fast-import: add helper function for inserting mark object entries
Currently, everything we want to insert into a mark set is an object
entry. However, in the future, we will want to insert objects of other
types. Teach read_mark_file to take a function pointer which helps us
insert the object we want into our mark set.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>