idxfile: optimise allocations in readObjectNames
idxfile: optimise allocations in readObjectNames

This makes all the required Entry allocations in one go,
instead of huge amounts of small individual allocations.

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

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