Added comment on top of lock file

This will warn people to not edit lock file manually.

Resolves #615

Changes

Author: gregory-m

cmd/dep/testdata/harness_tests/ensure/empty/case1/final/Gopkg.lock

@@ -13,7 +13,6 @@ import (
 	"syscall"
 
 	"github.com/golang/dep/internal"
-	"github.com/pelletier/go-toml"
 	"github.com/pkg/errors"
 )
 
@@ -61,22 +60,6 @@ func IsNonEmptyDir(name string) (bool, error) {
 	}
 }
 
-func writeFile(path string, in toml.Marshaler) error {
-	f, err := os.Create(path)
-	if err != nil {
-		return err
-	}
-	defer f.Close()
-
-	s, err := in.MarshalTOML()
-	if err != nil {
-		return err
-	}
-
-	_, err = f.Write(s)
-	return err
-}
-
 // modifyWithString modifies a given file with a new string input.
 // This is used to write arbitrary string data to a file, such as
 // updating the `Gopkg.toml` file with example data if no deps found

cmd/dep/testdata/harness_tests/ensure/empty/case2/final/Gopkg.lock

@@ -1,3 +1,5 @@
+# This file is autogenerated, do not edit; changes may be undone by the next 'dep ensure'.
+
 memo = "595716d270828e763c811ef79c9c41f85b1d1bfbdfe85280036405c03772206c"
 
 [[projects]]

cmd/dep/testdata/harness_tests/ensure/empty/case3/final/Gopkg.lock

@@ -22,7 +22,7 @@ import (
 // Example string to be written to the manifest file
 // if no dependencies are found in the project
 // during `dep init`
-const exampleTOML = `
+var exampleTOML = []byte(`
 ## Gopkg.toml example (these lines may be deleted)
 
 ## "required" lists a set of packages (not projects) that must be included in
@@ -74,7 +74,12 @@ const exampleTOML = `
 # source = "https://github.com/myfork/package.git"
 
 
-`
+`)
+
+// String added on top of lock file
+var lockFileComment = []byte(`# This file is autogenerated, do not edit; changes may be undone by the next 'dep ensure'.
+
+`)
 
 // SafeWriter transactionalizes writes of manifest, lock, and vendor dir, both
 // individually and in any combination, into a pseudo-atomic action with
@@ -307,22 +312,25 @@ func (sw *SafeWriter) Write(root string, sm gps.SourceManager, noExamples bool)
 			return errors.Wrap(err, "failed to marshal manifest to TOML")
 		}
 
-		var initOutput string
+		var initOutput []byte
 
 		// If examples are NOT disabled, use the example text
 		if !noExamples {
 			initOutput = exampleTOML
 		}
 
-		// 0666 is before umask; mirrors behavior of os.Create (used by
-		// writeFile())
-		if err = ioutil.WriteFile(filepath.Join(td, ManifestName), append([]byte(initOutput), tb...), 0666); err != nil {
+		if err = ioutil.WriteFile(filepath.Join(td, ManifestName), append(initOutput, tb...), 0666); err != nil {
 			return errors.Wrap(err, "failed to write manifest file to temp dir")
 		}
 	}
 
 	if sw.HasLock() {
-		if err := writeFile(filepath.Join(td, LockName), sw.lock); err != nil {
+		l, err := sw.lock.MarshalTOML()
+		if err != nil {
+			return errors.Wrap(err, "failed to marshal lock to TOML")
+		}
+
+		if err = ioutil.WriteFile(filepath.Join(td, LockName), append(lockFileComment, l...), 0666); err != nil {
 			return errors.Wrap(err, "failed to write lock file to temp dir")
 		}
 	}