NewRepository(): verify that the repository exists

Verify that the Git repository exists and that `git rev-parse
--git-dir` runs correctly. Distinguish between failures to run Git at
all (probably because it was not in the path) versus failures *of*
git (probably because the Git repository couldn't be found).

Author: mhagger

git-sizer.go

@@ -43,7 +43,7 @@ func (v *NegatedBoolValue) IsBoolFlag() bool {
 func main() {
 	err := mainImplementation()
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "%s\n", err)
+		fmt.Fprintf(os.Stderr, "error: %s\n", err)
 		os.Exit(1)
 	}
 }

sizes/git.go

@@ -57,9 +57,35 @@ type Repository struct {
 }
 
 func NewRepository(path string) (*Repository, error) {
-	return &Repository{
-		path: path,
-	}, nil
+	command := exec.Command(
+		"git", "-C", path,
+		"rev-parse", "--git-dir",
+	)
+	out, err := command.Output()
+	if err != nil {
+		switch err := err.(type) {
+		case *exec.Error:
+			return nil, errors.New(
+				fmt.Sprintf(
+					"could not run git (is it in your PATH?): %s",
+					err.Err,
+				),
+			)
+		case *exec.ExitError:
+			return nil, errors.New(
+				fmt.Sprintf(
+					"git rev-parse failed: %s",
+					err.Stderr,
+				),
+			)
+		default:
+			return nil, err
+		}
+	}
+	repo := &Repository{
+		path: string(bytes.TrimSpace(out)),
+	}
+	return repo, nil
 }
 
 func (repo *Repository) Close() error {