pack-objects: don't traverse objects unnecessarily

pack-objects: don't traverse objects unnecessarily

This brings back some of the performance lost in optimizing recency
order inside pack objects. We were doing extreme amounts of object
re-traversal: for the 2.14 million objects in the Linux kernel
repository, we were calling add_to_write_order() over 1.03 billion times
(a 0.2% hit rate, making 99.8% of of these calls extraneous).

Two optimizations take place here- we can start our objects array
iteration from a known point where we left off before we started trying
to find our tags, and we don't need to do the deep dives required by
add_family_to_write_order() if the object has already been marked as
filled.

These two optimizations bring some pretty spectacular results via `perf
stat`:

task-clock:   83373 ms        --> 43800 ms         (50% faster)
cycles:       221,633,461,676 --> 116,307,209,986  (47% fewer)
instructions: 149,299,179,939 --> 122,998,800,184  (18% fewer)

Helped-by: Ramsay Jones (format string fix in "die" message)
Signed-off-by: Dan McGee <dpmcgee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>