Description of problem:
When user selects the first version of a process in the Designer (via Overview -> Version history), the editor freezes, fails to load. All subsequent attempts at opening a process in the Designer fail. The server log shows a ConcurrentRefUpdateException from jgit.

Version-Release number of selected component (if applicable):
6.2 ER3

Steps to Reproduce:
1. Create a new business process.
2. Add an end node to the start node already present. Save and close the Designer.
3. Reopen the process in the Designer, switch to Overview, Version History.
4. Select the first version of the process.

Actual results:
Designer switches to the Editor tab but the process does not load. Closing and reopening the process does not help. No other process opens in the Designer.
Server log contains ConcurrentRefUpdateException.

Expected results:
Designer loads and displays correctly the first version of the process, without getting stuck. There is no exception in the server log, other processes can be opened in the Designer.