Make git-nav-downstream not print a stack if you Ctrl-C it.

R=agable@chromium.org
BUG=

Review URL: https://codereview.chromium.org/277003002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@269661 0039d316-1c4b-4281-b951-d872f2087c98

git_nav_downstream.py

@@ -58,4 +58,7 @@ def main(args):
 
 
 if __name__ == '__main__':
-  sys.exit(main(sys.argv[1:]))
+  try:
+    sys.exit(main(sys.argv[1:]))
+  except KeyboardInterrupt:
+    pass