merge: include a benchmark for lineCount
merge: include a benchmark for lineCount

Timings on our master branch:

$ go test -bench BenchmarkMergeFiles__lineCount
BenchmarkMergeFiles__lineCount-16    	      51	  41916935 ns/op

moov-io/ach,moov-io/ach,moov-io/ach,moov-io/ach