Better git fetch (#2626)

Related to https://github.com/FreshRSS/FreshRSS/pull/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 @@ class FreshRSS_update_Controller extends Minz_ActionController {
 		}
 		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() {