maintner: fix un-initialized github panic

Previously processGithubIssueMutation called initGithub, but
processGithubMutation did not, so if your on-disk mutation file had
a GithubMutation before a GithubIssueMutation, c.github would be nil
and Initialize() would panic.

Call initGithub from processGithubMutation and add a test to protect
against regressions.

Change-Id: Ie706fa04cb8ea87c2e0259dbee024c0005f0523f
Reviewed-on: https://go-review.googlesource.com/41298
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

maintner/github.go

@@ -62,6 +62,9 @@ func (g *GitHub) Repo(owner, repo string) *GitHubRepo {
 }
 
 func (g *GitHub) getOrCreateRepo(owner, repo string) *GitHubRepo {
+	if g == nil {
+		panic("cannot call methods on nil GitHub")
+	}
 	id := GithubRepoID{owner, repo}
 	if !id.valid() {
 		return nil
@@ -956,6 +959,10 @@ func (r *GitHubRepo) missingIssues() []int32 {
 
 // processGithubMutation updates the corpus with the information in m.
 func (c *Corpus) processGithubMutation(m *maintpb.GithubMutation) {
+	if c == nil {
+		panic("nil corpus")
+	}
+	c.initGithub()
 	gr := c.github.getOrCreateRepo(m.Owner, m.Repo)
 	if gr == nil {
 		log.Printf("bogus Owner/Repo %q/%q in mutation: %v", m.Owner, m.Repo, m)

maintner/maintner_test.go

@@ -112,6 +112,25 @@ func TestProcessMutation_Github_NewIssue(t *testing.T) {
 	})
 }
 
+func TestProcessMutation_Github(t *testing.T) {
+	c := NewCorpus(&dummyMutationLogger{}, "")
+	github := &GitHub{c: c}
+	c.github = github
+	github.repos = map[GithubRepoID]*GitHubRepo{
+		GithubRepoID{"golang", "go"}: &GitHubRepo{
+			github: github,
+			id:     GithubRepoID{"golang", "go"},
+			issues: make(map[int32]*GitHubIssue),
+		},
+	}
+	mutationTest{want: c}.test(t, &maintpb.Mutation{
+		Github: &maintpb.GithubMutation{
+			Owner: "golang",
+			Repo:  "go",
+		},
+	})
+}
+
 func TestNewMutationsFromIssue(t *testing.T) {
 	gh := &github.Issue{
 		Number:    github.Int(5),