Force creating pull request on GitHub

The hub command does not properly handle different pull and push
locations, so it fails to check that we've already pushed changes to
branch we want to pull.

Fixes #762
Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/vcs.py

@@ -651,6 +651,7 @@ class GithubRepository(GitRepository):
         """
         cmd = [
             'pull-request',
+            '-f',
             '-h', '{0}:{1}'.format(self._hub_user, fork_branch),
             '-b', origin_branch,
             '-m', 'Update from Weblate.'.encode('utf-8'),