go.tools/dashboard/builder: use "hg archive -t files -r REV dest" instead of "hg clone" for the work directory.

It will save a lot of time for slow builders running on slow SD cards.

LGTM=dave, adg
R=golang-codereviews, dave, adg
CC=golang-codereviews
https://golang.org/cl/63550043

builder/env.go

@@ -117,8 +117,11 @@ func (b *Builder) envvWindows() []string {
 // and returns the path workpath/go/src, the location of all go build scripts.
 func (env *goEnv) setup(repo *Repo, workpath, hash string, envv []string) (string, error) {
 	goworkpath := filepath.Join(workpath, "go")
-	if _, err := repo.Clone(goworkpath, hash); err != nil {
+	if err := repo.Export(goworkpath, hash); err != nil {
-		return "", fmt.Errorf("error cloning repository: %s", err)
+		return "", fmt.Errorf("error exporting repository: %s", err)
+	}
+	if err := ioutil.WriteFile(filepath.Join(goworkpath, "VERSION"), []byte(hash), 0644); err != nil {
+		return "", fmt.Errorf("error writing VERSION file: %s", err)
 	}
 	return filepath.Join(goworkpath, "src"), nil
 }

builder/vcs.go

@@ -61,6 +61,24 @@ func (r *Repo) Clone(path, rev string) (*Repo, error) {
 	}, nil
 }
 
+// Export exports the current Repo at revision rev to a new destination.
+func (r *Repo) Export(path, rev string) error {
+	r.Lock()
+	defer r.Unlock()
+
+	downloadPath := r.Path
+	if !r.Exists() {
+		_, err := r.Clone(path, rev)
+		return err
+	}
+
+	args := []string{r.Master.VCS.Cmd, "archive", "-t", "files", "-r", rev, path}
+	if err := run(*cmdTimeout, nil, downloadPath, args...); err != nil {
+		return fmt.Errorf("executing %s: %v", strings.Join(args, " "), err)
+	}
+	return nil
+}
+
 // UpdateTo updates the working copy of this Repo to the
 // supplied revision.
 func (r *Repo) UpdateTo(hash string) error {