Use git config instead of git remote set-branches

This allows us to work with older Git versions.

Closes #399
Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/vcs.py

@@ -439,14 +439,11 @@ class GitRepository(Repository):
             self.execute(['remote', 'set-url', 'origin', '--push', push_url])
 
         # Set branch to track
-        try:
-            self.execute(
-                ['remote', 'set-branches', 'origin', branch]
-            )
-        except RepositoryException:
-            self.execute(
-                ['remote', 'set-branches', '--add', 'origin', branch]
-            )
+        self.execute([
+            'config',
+            'remote.origin.fetch',
+            '+refs/heads/{0}:refs/remotes/origin/{0}'.format(branch)
+        ])
 
     def configure_branch(self, branch):
         """