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/tree_builder.py

@@ -104,14 +104,23 @@ class TreeBuilder:
         """For a given revision in the target, find the node in the source chain
         that matches the files"""
         node = source_chain
+        candidates = []
         while node:
             # exclude reverts happening after the merge
             if (
                 node.revision.commit_time <= revision.commit_time
                 and node.revision.files_hash == revision.files_hash
             ):
-                return node
+                candidates.append(node)
+            if node.merged_into:
+                # we can't have candidates that are crossing previous merges
+                # see https://gitea.opensuse.org/importers/git-importer/issues/14
+                candidates = []
             node = node.parent
+        if candidates:
+            # the assert is here to see if we have more hot cases
+            assert len(candidates) == 1
+            return candidates[0]
 
     def add_merge_points(self, factory_revisions):
         """For all target revisions that accepted a request, look up the merge