check for the output directory before trying to run any commands

Signed-off-by: Doug Hellmann <doug@doughellmann.com>

git_nit/cmd.py

@@ -16,7 +16,9 @@ from __future__ import print_function
 
 import argparse
 import json
+import os
 import subprocess
+import sys
 
 import pkg_resources
 import requests
@@ -115,6 +117,9 @@ def main():
     else:
         clone_to = '{}-{}-{}'.format(short_repo, review, subject)
 
+    if os.path.exists(clone_to):
+        sys.exit('{} already exists'.format(clone_to))
+
     git_cmd = [
         'git',
         'clone',