Ignore merge point candidates that create crosses

In OBS you can create submit requests for revisions that are behind
the last merge point, in git you can't - so we ignore them.

Fixes #14

lib/db_revision.py

@@ -31,6 +31,7 @@ class DBRevision:
         self.rev = float(self.rev)
         self._files = None
         self.db = db
+        self.git_commit = None
 
     def short_string(self):
         return f"{self.project}/{self.package}/{self.rev}"