Allow stubmaker to work outside of a git repository (#24678)

Co-authored-by: Violet Hynes <violet.hynes@hashicorp.com>

tools/stubmaker/main.go

@@ -50,16 +50,22 @@ func main() {
 		DetectDotGit: true,
 	})
 	if err != nil {
+		if err.Error() != "repository does not exist" {
 			fatal(err)
 		}
+		repo = nil
+	}
 
-	wt, err := repo.Worktree()
+	var wt *git.Worktree
+	if repo != nil {
+		wt, err = repo.Worktree()
 		if err != nil {
 			fatal(err)
 		}
 		if !isEnterprise(wt) {
 			return
 		}
+	}
 
 	// Read the file and figure out if we need to do anything.
 	inputFile := os.Getenv("GOFILE")
@@ -88,6 +94,7 @@ func main() {
 
 	// We'd like to write the file, but first make sure that we're not going
 	// to blow away anyone's work or overwrite a file already in git.
+	if repo != nil {
 		head, err := repo.Head()
 		if err != nil {
 			fatal(err)
@@ -109,6 +116,7 @@ func main() {
 		if tracked {
 			fatal(fmt.Errorf("output file %s exists in git, not overwriting", outputFile))
 		}
+	}
 
 	// Now we can finally write the file
 	output, err := os.Create(outputFile + ".tmp")