Simplify. NFCI

Co-authored-by: David Leadbeater <dgl@dgl.cx>

src/fetch.go

@@ -146,7 +146,8 @@ func FetchRepository(
 	for _, oldManifestEntry := range oldManifest.GetContents() {
 		if hash, ok := plumbing.FromHex(oldManifestEntry.GetGitHash()); ok {
 			if manifestEntry, found := blobsNeeded[hash]; found {
-				CopyProtoMessage(manifestEntry, oldManifestEntry)
+				manifestEntry.Reset()
+				proto.Merge(manifestEntry, oldManifestEntry)
 				dataBytesFromOldManifest += oldManifestEntry.GetOriginalSize()
 				delete(blobsNeeded, hash)
 			}

src/util.go

@@ -4,8 +4,6 @@ import (
 	"errors"
 	"io"
 	"strings"
-
-	"google.golang.org/protobuf/proto"
 )
 
 type BoundedReader struct {
@@ -87,19 +85,3 @@ func getMediaType(mimeType string) (mediaType string) {
 	mediaType = strings.TrimSpace(strings.ToLower(mediaType))
 	return
 }
-
-// Copying Protobuf messages like `*dest = *src` causes a lock to be copied, which is unsound.
-// Copying Protobuf messages field-wise is fragile: adding a new field to the schema does not
-// cause a diagnostic to be emitted pointing to the copy site, making it easy to miss updates.
-// Serializing and deserializing is reliable and breaks referential links.
-func CopyProtoMessage(dest, src proto.Message) {
-	data, err := proto.Marshal(src)
-	if err != nil {
-		panic(err)
-	}
-
-	err = proto.Unmarshal(data, dest)
-	if err != nil {
-		panic(err)
-	}
-}