benchstat: skip empty rows when just dumping data

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

benchstat/table.go

@@ -75,6 +75,20 @@ func (c *Collection) Tables() []*Table {
 					}
 				}
 
+				// if data for this unit is all missing - omit row entirely.
+				missing := true
+				for _, cfg := range c.Configs {
+					k := key
+					k.Config = cfg
+					if c.Metrics[k] != nil {
+						missing = false
+						break
+					}
+				}
+				if missing {
+					continue
+				}
+
 				// If there are only two configs being compared, add stats.
 				if table.OldNewDelta {
 					k0 := key

cmd/benchstat/main_test.go

@@ -39,6 +39,8 @@ func TestGolden(t *testing.T) {
 	check(t, "packagesold", "packagesold.txt")
 	check(t, "packages", "packagesold.txt", "packagesnew.txt")
 	check(t, "units", "units-old.txt", "units-new.txt")
+	check(t, "units1", "units-old.txt")
+	check(t, "units3", "units-old.txt", "units-new.txt", "units-new.txt")
 	check(t, "zero", "-delta-test=none", "zero-old.txt", "zero-new.txt")
 }
 

cmd/benchstat/testdata/units1.golden

+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%

cmd/benchstat/testdata/units3.golden

+name \ time/op       units-old.txt  units-new.txt  units-new.txt
+TwoHourMarathon         7200s ± 0%     7200s ± 0%     7200s ± 0%
+Timens                 31.0ns ± 0%    31.0ns ± 0%    31.0ns ± 0%
+Timeμs                 32.0µs ± 0%    32.0µs ± 0%    32.0µs ± 0%
+Timeµs                 33.0µs ± 0%    33.0µs ± 0%    33.0µs ± 0%
+Timeus                 34.0µs ± 0%    34.0µs ± 0%    34.0µs ± 0%
+Timems                 35.0ms ± 0%    35.0ms ± 0%    35.0ms ± 0%
+Times                   36.0s ± 0%     36.0s ± 0%     36.0s ± 0%
+
+name \ user-time/op  units-old.txt  units-new.txt  units-new.txt
+TwoHourMarathon        14400s ± 1%    14380s ± 3%    14380s ± 3%
+Timens                 21.0ns ± 0%    21.0ns ± 0%    21.0ns ± 0%
+Timeμs                 22.0µs ± 0%    22.0µs ± 0%    22.0µs ± 0%
+Timeµs                 23.0µs ± 0%    23.0µs ± 0%    23.0µs ± 0%
+Timeus                 24.0µs ± 0%    24.0µs ± 0%    24.0µs ± 0%
+Timems                 25.0ms ± 0%    25.0ms ± 0%    25.0ms ± 0%
+Times                   26.0s ± 0%     26.0s ± 0%     26.0s ± 0%
+
+name \ time/GC       units-old.txt  units-new.txt  units-new.txt
+TwoHourMarathon        5.00ns ± 0%    5.00ns ± 0%    5.00ns ± 0%
+
+name \ quick-bytes   units-old.txt  units-new.txt  units-new.txt
+TwoHourMarathon         13.6B ±18%     13.6B ±18%     13.6B ±18%
+
+name \ time/object   units-old.txt  units-new.txt  units-new.txt
+TimeObjectns           41.0ns ± 0%    41.0ns ± 0%    41.0ns ± 0%
+TimeObjectµs           42.0µs ± 0%    42.0µs ± 0%    42.0µs ± 0%
+TimeObjectus           43.0µs ± 0%    43.0µs ± 0%    43.0µs ± 0%
+TimeObjectms           44.0ms ± 0%    44.0ms ± 0%    44.0ms ± 0%
+TimeObjects             45.0s ± 0%     45.0s ± 0%     45.0s ± 0%