Fix fetch --dry-run to not invoke git

This was broken by 209a681f9c,
which replaced self.run() with git_common.run(), thus
bypassing self.run()'s dry_run check.

Bug: 788562
Change-Id: I9964c169b0ff56778c4660acab7ef7c010e37a36
Reviewed-on: https://chromium-review.googlesource.com/797611
Commit-Queue: Aaron Gable <agable@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>

fetch.py

@@ -91,6 +91,8 @@ class GitCheckout(Checkout):
 
   def run_git(self, *cmd, **kwargs):
     print 'Running: git %s' % (' '.join(pipes.quote(x) for x in cmd))
+    if self.options.dry_run:
+      return ''
     return git_common.run(*cmd, **kwargs)