encoding: add Format helper function and method

The Format function and MarshalOptions.Format method are helper
functions for directly obtaining the formatted string for a message
without having to deal with errors or convert a []byte to string.
It is only intended for human consumption (e.g., debugging or logging).

We also add a MarshalOptions.Multiline option to specify that the output
should use some default indentation in a multiline output.

This assists in the v1 to v2 migration where:
	protoV1.CompactTextString(m) => prototext.MarshalOptions{}.Format(m)
	protoV1.MarshalTextString(m) => prototext.Format(m)

At Google, there are approximately 10x more usages of MarshalTextString than
CompactTextString, so it makes sense that the top-level Format function
does multiline expansion by default.

Fixes #850

Change-Id: I149c9e190a6d99b985d3884df675499a3313e9b3
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/213460
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/proto/isinit_test.go b/proto/isinit_test.go
index 6a3a8c9..e38b76d 100644
--- a/proto/isinit_test.go
+++ b/proto/isinit_test.go
@@ -9,6 +9,7 @@
 	"strings"
 	"testing"
 
+	"google.golang.org/protobuf/encoding/prototext"
 	"google.golang.org/protobuf/internal/flags"
 	"google.golang.org/protobuf/proto"
 
@@ -81,7 +82,7 @@
 				got = fmt.Sprintf("%q", err)
 			}
 			if !strings.Contains(got, tt.want) {
-				t.Errorf("IsInitialized(m):\n got: %v\nwant contains: %v\nMessage:\n%v", got, tt.want, marshalText(tt.m))
+				t.Errorf("IsInitialized(m):\n got: %v\nwant contains: %v\nMessage:\n%v", got, tt.want, prototext.Format(tt.m))
 			}
 		})
 	}