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/git_exporter.py

@@ -107,10 +107,12 @@ class GitExporter:
         if flat.parent1:
             if not self.branch_fits_parent1(flat, branch_state):
                 logging.info(f"Reset {flat.branch} onto {flat.parent1.short_string()}")
+                assert flat.parent1.git_commit
                 self.git.set_branch_head(flat.branch, flat.parent1.git_commit)
                 self.git.checkout(flat.branch)
             parents.append(flat.parent1.git_commit)
             if flat.parent2:
+                assert flat.parent2.git_commit
                 parents.append(flat.parent2.git_commit)
 
         # Overwrite ".gitattributes" with the