Add gitattributes file to hide autogen code (#131)

Hide autogenerated code from PRs. I renamed the go-helpers files while I
was here so that they used the common '.pb.go' suffix used by other
protoc plugins

.gitattributes

@@ -0,0 +1,9 @@
+# vendor is marked as linguist-vendored by default, but this
+# only excluses the code from stats.
+#
+# Forcing it to be linguist-generated will also exclude it from diffs.
+#
+# See https://github.com/github-linguist/linguist/blob/master/docs/overrides.md#summary
+# for more info.
+*.pb.go linguist-generated
+*.mock.go linguist-generated

cmd/protoc-gen-go-helpers/main.go

@@ -195,7 +195,7 @@ func main() {
 				return fmt.Errorf("failed to format generated source: \n%s\n%w", src.String(), err)
 			}
 
-			gf := plugin.NewGeneratedFile(fmt.Sprintf("%s.go-helpers.go", file.GeneratedFilenamePrefix), ".")
+			gf := plugin.NewGeneratedFile(fmt.Sprintf("%s.go-helpers.pb.go", file.GeneratedFilenamePrefix), ".")
 			gf.Write(fmtd)
 		}