This reduces the rate of syscalls for the common case of sequences of
object reads from the same pack.

Best of 5 timings for libgit2_clar before this patch:
real    0m5.375s
user    0m0.392s
sys     0m3.564s

After applying this patch:
real    0m5.285s
user    0m0.356s
sys     0m3.544s

0.6% improvement in system time.
9.2% improvement in user time.
1.7% improvement in elapsed time.

Confirmed a 0.6% reduction in number of system calls with strace.

Expect greater improvement for graph-traversal with large packs.