[IMP] runbot: use a separate method for git fetch command

At the end of the _update_git method, the "git fetch" command is run.
That makes it diffcult to override to change its behavior (for example
to avoid fetching pull requests).

With this commit, the command is separated in a new small method that
can be easily overriden.

runbot/models/repo.py

@@ -291,7 +291,12 @@ class runbot_repo(models.Model):
                 _logger.debug('repo %s skip hook fetch fetch_time: %ss ago hook_time: %ss ago',
                               repo.name, int(t0 - fetch_time), int(t0 - dt2time(repo.hook_time)))
                 return
+        self._update_fetch_cmd()
 
+    def _update_fetch_cmd(self):
+        # Extracted from update_git to be easily overriden in external module
+        self.ensure_one()
+        repo = self
         repo._git(['fetch', '-p', 'origin', '+refs/heads/*:refs/heads/*', '+refs/pull/*/head:refs/pull/*'])
 
     @api.multi