analysis: Detect "Code generated by" header.

This is in line with https://golang.org/cl/15073.

Consider .go files that are shorter than 2 lines (i.e., no "\n" character)
to not be generated.

analysis/generated_detection.go

@@ -2,6 +2,7 @@ package analysis
 
 import (
 	"bufio"
+	"io"
 	"os"
 	"path/filepath"
 	"strings"
@@ -22,10 +23,13 @@ func IsFileGenerated(rootDir, name string) (bool, error) {
 	defer f.Close()
 	r := bufio.NewReader(f)
 	s, err := r.ReadString('\n')
-	if err != nil {
+	if err == io.EOF {
+		// Empty file or exactly 1 line is not considered to be generated.
+		return false, nil
+	} else if err != nil {
 		return false, err
 	}
-	if s == "// Code generated by protoc-gen-gogo.\n" { // protoc-gen-gogo special case.
+	if strings.Contains(s, "Code generated by") { // Consistent with https://golang.org/cl/15073.
 		return true, nil
 	}
 	return (strings.Contains(s, "GENERATED") || strings.Contains(s, "generated")) && strings.Contains(s, "DO NOT EDIT"), nil

analysis/generated_detection_test.go

@@ -15,8 +15,10 @@ func ExampleIsFileGenerated() {
 
 	fmt.Println(analysis.IsFileGenerated(cwd, "testdata/generated_0.go"))
 	fmt.Println(analysis.IsFileGenerated(cwd, "testdata/handcrafted_0.go"))
+	fmt.Println(analysis.IsFileGenerated(cwd, "testdata/handcrafted_1.go"))
 
 	// Output:
 	// true <nil>
 	// false <nil>
+	// false <nil>
 }

analysis/testdata/handcrafted_1.go

@@ -0,0 +1 @@
+// Code generated by protoc-gen-gogo. Actually it isn't, because it's only 1 line.