Add GeneratePRTemplate method to github/template

The function of this method is to defer the logic of generating a commit
message to template.go, and eventually remove that logic from
commands/pull_request.go

Also adds tests for this new method, which also modifies the
fixtures/test_repo.go file to accomidate a test repo with a `.github` PR
dir.

fixtures/test_repo.go

@@ -59,6 +59,31 @@ func (r *TestRepo) AddRemote(name, url, pushURL string) {
 	}
 }
 
+func (r *TestRepo) AddGithubTemplatesDir() {
+	github_dir := filepath.Join(r.dir, "test.git", ".github")
+	pr_template_path := filepath.Join(github_dir, "PULL_REQUEST_TEMPLATE.md")
+	issue_template_path := filepath.Join(github_dir, "ISSUE_TEMPLATE")
+
+	// Make `.github` dir in root of test repo
+	err := os.MkdirAll(filepath.Dir(issue_template_path), 0771)
+	if err != nil {
+		panic(err)
+	}
+
+	// Switch to the root of the project dir
+	err = os.Chdir(filepath.Join(r.dir, "test.git"))
+	if err != nil {
+		panic(err)
+	}
+
+	content := `Description
+-----------
+[Enter your pull request description here]
+`
+	ioutil.WriteFile(pr_template_path, []byte(content), os.ModePerm)
+	ioutil.WriteFile(issue_template_path, []byte(content), os.ModePerm)
+}
+
 func (r *TestRepo) clone(repo, dir string) error {
 	cmd := cmd.New("git").WithArgs("clone", repo, dir)
 	output, err := cmd.CombinedOutput()

github/template.go

@@ -20,7 +20,11 @@ func GetPullRequestTemplate() string {
 }
 
 func GetIssueTemplate() string {
-	return getGithubTemplate(issueTempalte)
+	return getGithubTemplate(issueTemplate)
+}
+
+func GeneratePRTemplate(defaultMsg string) string {
+	return "\n\n" + GetPullRequestTemplate()
 }
 
 func getGithubTemplate(pat string) (body string) {

github/template_test.go

@@ -0,0 +1,27 @@
+package github
+
+import (
+	"testing"
+
+	"github.com/bmizerany/assert"
+	"github.com/github/hub/fixtures"
+)
+
+// When no default message is provided, two blank lines should be added
+// (representing the pull request title), and the left should be template.
+func TestGeneratePRTemplate_NoDefaultMessage(t *testing.T) {
+	repo := fixtures.SetupTestRepo()
+	defer repo.TearDown()
+
+	repo.AddGithubTemplatesDir()
+
+	defaultMessage := ""
+	expectedOutput := `
+
+Description
+-----------
+[Enter your pull request description here]
+`
+
+	assert.Equal(t, expectedOutput, GeneratePRTemplate(defaultMessage))
+}