unpack-trees: fix accidentally quadratic behavior

unpack-trees: fix accidentally quadratic behavior

While unpacking trees (e.g. during git checkout), when we hit a cache
entry that's past and outside our path, we cut off iteration.

This provides about a 45% speedup on git checkout between master and
master^20000 on Twitter's monorepo.  Speedup in general will depend on
repostitory structure, number of changes, and packfile packing
decisions.

Signed-off-by: David Turner <redacted>
Signed-off-by: Junio C Hamano <redacted>