svn: allow legacy (pre SVN 1.7) extraction of post commit data.

- This allows pushing without errors, but limits integration functionality
- Better don't raise exceptions here, to allow older systems to push
- log.error about it to inform users.

vcsserver/hooks.py

@@ -691,7 +691,14 b' def svn_post_commit(repo_path, commit_da'
     """
     commit_data is path, rev, txn_id
     """
+    if len(commit_data) == 3:
     path, commit_id, txn_id = commit_data
+    elif len(commit_data) == 2:
+        log.error('Failed to extract txn_id from commit_data using legacy method. '
+                  'Some functionality might be limited')
+        path, commit_id = commit_data
+        txn_id = None
+
     branches = []
     tags = []