Forcibly export fields for use by the reporter

The choice to avoid traversing unexported fields by default makes
sense for the semantics of normal comparisons. However, the goal
of the reporter is to prioritize humanly readable output.
As such, it seems appropriate to forcibly export fields.
This allows the String method to be called on such values.

cmp/report_reflect.go

@@ -127,13 +127,18 @@ func (opts formatOptions) FormatValue(v reflect.Value, withinSlice bool, m visit
 		return textLine(formatPointer(v))
 	case reflect.Struct:
 		var list textList
+		v := makeAddressable(v) // needed for retrieveUnexportedField
 		for i := 0; i < v.NumField(); i++ {
 			vv := v.Field(i)
 			if value.IsZero(vv) {
 				continue // Elide fields with zero values
 			}
+			sf := t.Field(i)
+			if supportExporters && !isExported(sf.Name) {
+				vv = retrieveUnexportedField(v, sf)
+			}
 			s := opts.WithTypeMode(autoType).FormatValue(vv, false, m)
-			list = append(list, textRecord{Key: t.Field(i).Name, Value: s})
+			list = append(list, textRecord{Key: sf.Name, Value: s})
 		}
 		return textWrap{"{", list, "}"}
 	case reflect.Slice:

cmp/testdata/diffs

@@ -1154,10 +1154,10 @@
   	CleanGerms: nil,
   	GermMap:    map[int32]*testprotos.Germ{13: s"germ13", 21: s"germ21"},
   	DishMap: map[int32]*teststructs.Dish{
-  		0: &{err: &errors.errorString{s: "EOF"}},
+  		0: &{err: e"EOF"},
 - 		1: nil,
-+ 		1: &{err: &errors.errorString{s: "unexpected EOF"}},
-  		2: &{pb: &testprotos.Dish{Stringer: testprotos.Stringer{X: "dish"}}},
++ 		1: &{err: e"unexpected EOF"},
+  		2: &{pb: s"dish"},
   	},
   	HasPreviousResult: true,
   	DirtyID:           10,