Improve formatting of durations

Since "m" could stand for both "minute" and "month",
and months vary in length, let's stop at days.

acid_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"testing"
 	ttemplate "text/template"
+	"time"
 )
 
 func TestTemplateQuote(t *testing.T) {
@@ -30,3 +31,19 @@ func TestTemplateQuote(t *testing.T) {
 		}
 	}
 }
+
+func TestShortDurationString(t *testing.T) {
+	for _, test := range []struct {
+		d      time.Duration
+		expect string
+	}{
+		{72 * time.Hour, "3d"},
+		{-3 * time.Hour, "-3h"},
+		{12 * time.Minute, "12m"},
+		{time.Millisecond, "0s"},
+	} {
+		if sd := shortDurationString(test.d); sd != test.expect {
+			t.Errorf("%s = %s; want %s\n", test.d, sd, test.expect)
+		}
+	}
+}