Improve performace for reading many small objects from packfile by le…

…tting the I/O buffers be used better.

plumbing/format/packfile/packfile.go

@@ -208,7 +208,7 @@ func (p *Packfile) objectAtOffset(offset int64) (plumbing.EncodedObject, error)
 
 	// If we have no filesystem, we will return a MemoryObject instead
 	// of an FSObject.
-	if p.fs == nil {
+	if p.fs == nil || h.Length <= 16 * 1024 {
 		return p.getNextObject(h)
 	}