packfile: optimise NewIndexFromIdxFile for a very common case
packfile: optimise NewIndexFromIdxFile for a very common case

Loading from an on-disk idxfile will usually already have the idxfile
entries in order, so check that before wasting time on sorting.

Signed-off-by: David Symonds <7f6bb564ed9e580d4f0aa90d53b34578e5d24cb9@golang.org>

go-git/go-git,src-d/go-git,mcuadros/go-git,go-git/go-git,src-d/go-git,mcuadros/go-git