initial work toward correctly merging deeper import histories

Pure code is tested working, including with even histories that merge
several lines of development. Needs to be hooked up to git histories
next.

doc/todo/import_tree.mdwn

@@ -18,6 +18,8 @@ and `git annex sync --content` can be configured to use it.
   when importing from a versioned S3 remote,
   although the head of it does reflect the current state of the remote.
 
+  Note: Merge commits may complicate this.
+
 * S3 buckets can be set up to allow reads and listing by an anonymous user.
   That should allow importing from such a bucket, but the S3 remote
   will need changes, since it currently avoids using the S3 API when