Description of problem:
Revert back to a previous version of a business process model causes the exception:

14:58:30,356 ERROR [stderr] (http-mackoy/127.0.0.1:8080-2) java.lang.RuntimeException: org.eclipse.jgit.api.errors.ConcurrentRefUpdateException: Could not lock HEAD. RefUpdate return code was: LOCK_FAILURE
14:58:30,357 ERROR [stderr] (http-mackoy/127.0.0.1:8080-2) 	at org.uberfire.java.nio.fs.jgit.util.JGitUtil.commit(JGitUtil.java:608)
14:58:30,357 ERROR [stderr] (http-mackoy/127.0.0.1:8080-2) 	at org.uberfire.java.nio.fs.jgit.util.JGitUtil.commit(JGitUtil.java:506)
14:58:30,357 ERROR [stderr] (http-mackoy/127.0.0.1:8080-2) 	at org.uberfire.java.nio.fs.jgit.JGitFileSystemProvider.commit(JGitFileSystemProvider.java:1853)
14:58:30,357 ERROR [stderr] (http-mackoy/127.0.0.1:8080-2) 	at org.uberfire.java.nio.fs.jgit.JGitFileSystemProvider.access$600(JGitFileSystemProvider.java:147)
14:58:30,357 ERROR [stderr] (http-mackoy/127.0.0.1:8080-2) 	at org.uberfire.java.nio.fs.jgit.JGitFileSystemProvider$4.close(JGitFileSystemProvider.java:836)
14:58:30,357 ERROR [stderr] (http-mackoy/127.0.0.1:8080-2) 	at org.uberfire.io.impl.AbstractIOService.write(AbstractIOService.java:609)

.. and the process is not restored.


Version-Release number of selected component (if applicable):
BPM Suite 6.1.0

How reproducible:
Always

Steps to Reproduce:
1. Create a process with a start node, task node and end node.
2. Save it adding a comment to the save operation and close the process.
3. Open it again and delete the end node.
4. Save it adding a different comment to the save operation and close the process again.
5. Open the process and go to Overview tab.
6. Select the version saved with the comment added in the step 2. The Designer will open that version, but the commit is not reverted, if closed the tab the current version will still be that one saved in step 4 - no problem - to save the restored process we need to click on Save again, and here we have two problems:
6.1. If a previous version is restored and is tried to save it, Designer will show the message "process contains no changes" and it will not be saved.
6.2. If a previous version is restored and a update is done and then is tried to save it, business-central will throw the exception:

"org.eclipse.jgit.api.errors.ConcurrentRefUpdateException: Could not lock HEAD. RefUpdate return code was: LOCK_FAILURE"

.. and it will not be saved.


Actual results:
See items 6.1 and 6.2 above

Expected results:
Process restored and saved without exceptions 

Additional info: