pack-objects: mark add_to_write_order() as inline

pack-objects: mark add_to_write_order() as inline

This function is a whole 26 bytes when compiled on x86_64, but is
currently invoked over 1.037 billion times when running pack-objects on
the Linux kernel git repository. This is hitting the point where
micro-optimizations do make a difference, and inlining it only increases
the object file size by 38 bytes.

As reported by perf, this dropped task-clock from 84183 to 83373 ms, and
total cycles from 223.5 billion to 221.6 billion. Not astronomical, but
worth getting for adding one word.

Signed-off-by: Dan McGee <dpmcgee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>