Better git fetch

Related to FreshRSS#2625

If for some reasons branches have diverged:, e.g.:

```
$ git status -sb --porcelain remote
## dev...origin/dev [ahead 4, behind 1]
```

app/Controllers/updateController.php

@@ -23,7 +23,7 @@ public static function hasGitUpdate() {
  }
  chdir($cwd);
  $line = is_array($output) ? implode('; ', $output) : '' . $output;
- return strpos($line, '[behind') !== false;
+ return strpos($line, '[behind') !== false || strpos($line, '[ahead') !== false;
  }
 
  public static function gitPull() {