When comparing 2 files, benchstat skips rows that are not present in
both benchmarks. However currently when benchstat'ing just 1 file or 3
or more, benchstat dumps every row for every unit, even if row results
for the unit are completely empty.

When several units are in use, this creates a lot of noise of empty
benchmark lines output. For example for the following benchmark

```
BenchmarkTwoHourMarathon 1 7200000000000 ns/op 14400000000000 user-ns/op 5 ns/GC 12 quick-bytes
BenchmarkTwoHourMarathon 1 7200000000000 ns/op 14500000000000 user-ns/op 5 ns/GC 16 quick-bytes
BenchmarkTwoHourMarathon 1 7200000000000 ns/op 14600000000000 user-ns/op 5 ns/GC 12 quick-bytes
BenchmarkTwoHourMarathon 1 7200000000000 ns/op 14200000000000 user-ns/op 5 ns/GC 16 quick-bytes
BenchmarkTwoHourMarathon 1 7200000000000 ns/op 14300000000000 user-ns/op 5 ns/GC 12 quick-bytes

BenchmarkTimens 1 31 ns/op 21 user-ns/op
BenchmarkTimeμs 1 32 μs/op 22 user-μs/op	# mu = U+03bc
BenchmarkTimeµs 1 33 µs/op 23 user-µs/op	# mu = U+b5
BenchmarkTimeus 1 34 us/op 24 user-us/op
BenchmarkTimems 1 35 ms/op 25 user-ms/op
BenchmarkTimes  1 36  s/op 26 user-s/op

BenchmarkTimeObjectns 1 41 ns/object
BenchmarkTimeObjectµs 1 42 µs/object
BenchmarkTimeObjectus 1 43 us/object
BenchmarkTimeObjectms 1 44 ms/object
BenchmarkTimeObjects  1 45 s/object
```

current output is

```
name             time/op
TwoHourMarathon    7200s ± 0%
Timens            31.0ns ± 0%
Timeμs            32.0µs ± 0%
Timeµs            33.0µs ± 0%
Timeus            34.0µs ± 0%
Timems            35.0ms ± 0%
Times              36.0s ± 0%
TimeObjectns
TimeObjectµs
TimeObjectus
TimeObjectms
TimeObjects

name             user-time/op
TwoHourMarathon   14400s ± 1%
Timens            21.0ns ± 0%
Timeμs            22.0µs ± 0%
Timeµs            23.0µs ± 0%
Timeus            24.0µs ± 0%
Timems            25.0ms ± 0%
Times              26.0s ± 0%
TimeObjectns
TimeObjectµs
TimeObjectus
TimeObjectms
TimeObjects

name             time/GC
TwoHourMarathon   5.00ns ± 0%
Timens
Timeμs
Timeµs
Timeus
Timems
Times
TimeObjectns
TimeObjectµs
TimeObjectus
TimeObjectms
TimeObjects

name             quick-bytes
TwoHourMarathon    13.6B ±18%
Timens
Timeμs
Timeµs
Timeus
Timems
Times
TimeObjectns
TimeObjectµs
TimeObjectus
TimeObjectms
TimeObjects

name             time/object
TwoHourMarathon
Timens
Timeμs
Timeµs
Timeus
Timems
Times
TimeObjectns      41.0ns ± 0%
TimeObjectµs      42.0µs ± 0%
TimeObjectus      43.0µs ± 0%
TimeObjectms      44.0ms ± 0%
TimeObjects        45.0s ± 0%
```

and it becomes

```
name             time/op
TwoHourMarathon    7200s ± 0%
Timens            31.0ns ± 0%
Timeμs            32.0µs ± 0%
Timeµs            33.0µs ± 0%
Timeus            34.0µs ± 0%
Timems            35.0ms ± 0%
Times              36.0s ± 0%

name             user-time/op
TwoHourMarathon   14400s ± 1%
Timens            21.0ns ± 0%
Timeμs            22.0µs ± 0%
Timeµs            23.0µs ± 0%
Timeus            24.0µs ± 0%
Timems            25.0ms ± 0%
Times              26.0s ± 0%

name             time/GC
TwoHourMarathon   5.00ns ± 0%

name             quick-bytes
TwoHourMarathon    13.6B ±18%

name             time/object
TimeObjectns      41.0ns ± 0%
TimeObjectµs      42.0µs ± 0%
TimeObjectus      43.0µs ± 0%
TimeObjectms      44.0ms ± 0%
TimeObjects        45.0s ± 0%
```

after this patch.

Change-Id: Ic260e39913af7b2c3aa1ff850aed65002d553d6c