git: return 0 from exitCode if err == nil

exit.go

@@ -16,9 +16,13 @@ package git
 
 import "errors"
 
-// exitCode returns the exit code indicated by the error, or -1 if the error
-// doesn't indicate an exited process.
+// exitCode returns the exit code indicated by the error,
+// zero if the error is nil,
+// or -1 if the error doesn't indicate an exited process.
 func exitCode(err error) int {
+	if err == nil {
+		return 0
+	}
 	var coder interface {
 		ExitCode() int
 	}

exit_test.go

@@ -44,6 +44,10 @@ func TestExitCode(t *testing.T) {
 			err:  errors.New("bork"),
 			want: -1,
 		},
+		{
+			err:  nil,
+			want: 0,
+		},
 	}
 	for _, test := range tests {
 		if got := exitCode(test.err); got != test.want {