fix: better output format for git command (#113)

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**What**:

Without the space, it looks like a little strange.

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src/util/git.js

@@ -41,7 +41,7 @@ const spawnGitCommand = pify((args, cb) => {
   git.stderr.on('data', (buf) => bufs.push(buf));
   git.on('close', (code) => {
     if (code) {
-      const msg = Buffer.concat(bufs).toString() || `git ${args.join('')} - exit code: ${code}`;
+      const msg = Buffer.concat(bufs).toString() || `git ${args.join(' ')} - exit code: ${code}`;
       cb(new Error(msg));
     } else {
       cb(null);