encoding/gob: revert 6348067, which broke compatibility

Add commentary to explain better what's going on, but the
code change is a simple one-line reversal to the previous
form.

R=rsc
CC=golang-dev
https://golang.org/cl/6428072

src/pkg/encoding/gob/type.go

@@ -749,13 +749,29 @@ func Register(value interface{}) {
 	rt := reflect.TypeOf(value)
 	name := rt.String()
 
-	// But for named types (or pointers to them), qualify with import path.
+	// But for named types (or pointers to them), qualify with import path (but see inner comment).
 	// Dereference one pointer looking for a named type.
 	star := ""
 	if rt.Name() == "" {
 		if pt := rt; pt.Kind() == reflect.Ptr {
 			star = "*"
-			rt = pt.Elem()
+			// NOTE: The following line should be rt = pt.Elem() to implement
+			// what the comment above claims, but fixing it would break compatibility
+			// with existing gobs.
+			//
+			// Given package p imported as "full/p" with these definitions:
+			//     package p
+			//     type T1 struct { ... }
+			// this table shows the intended and actual strings used by gob to
+			// name the types:
+			//
+			// Type      Correct string     Actual string
+			//
+			// T1        full/p.T1          full/p.T1
+			// *T1       *full/p.T1         *p.T1
+			//
+			// The missing full path cannot be fixed without breaking existing gob decoders.
+			rt = pt
 		}
 	}
 	if rt.Name() != "" {