autobuild: fixed clone source for build trees

this fixes a bug where autobuild would test the tree that the user
pushed, not the tree that has been rebased on master. That could cause
broken trees to get into master. Andrew Bartlett noticed this with a
tree from Jan 19th. 

Pair-Programmed-With: Andrew Bartlett <abartlet@samba.org>

script/autobuild.py

@@ -157,7 +157,7 @@ class builder(object):
         cleanup_list.append(self.prefix)
         os.makedirs(self.sdir)
         run_cmd("rm -rf %s" % self.sdir)
-        run_cmd("git clone --shared %s %s" % (gitroot, self.sdir))
+        run_cmd("git clone --shared %s %s" % (test_master, self.sdir), dir=test_master, show=True)
         self.start_next()
 
     def start_next(self):