EGit Merge tool doesn't work with XtextEditor

Steps to reproduce:

• create an Xtext project (with at least one Xtext document) and add it to a Git repository
• on master branch: change the Xtext document and commit it
• switch to another branch (cloned from master) and change the same Xtext document (with another modification which should provoke a merge conflict with master branch's file), commit it
• merge branch master into the current branch so that a merge conflict occurs
• use Team -> Merge Tool on that Xtext document
  The following exception is thrown:

Caused by: org.eclipse.ui.PartInitException: Editor could not be initialized.
	at org.eclipse.ui.texteditor.AbstractTextEditor.internalInit(AbstractTextEditor.java:3188)
	at org.eclipse.ui.texteditor.AbstractTextEditor.init(AbstractTextEditor.java:3197)
	at org.eclipse.xtext.ui.editor.XtextEditor.init(XtextEditor.java:305)
	at org.eclipse.xtext.ui.compare.DefaultMergeViewer.createSourceViewerConfiguration(DefaultMergeViewer.java:190)
	... 62 more
Caused by: java.lang.IllegalArgumentException: document may not be null
	at org.eclipse.xtext.ui.editor.DocumentBasedDirtyResource.connect(DocumentBasedDirtyResource.java:36)
	at org.eclipse.xtext.ui.editor.DirtyStateEditorSupport.initDirtyResource(DirtyStateEditorSupport.java:448)
	at org.eclipse.xtext.ui.editor.DirtyStateEditorSupport.initializeDirtyStateSupport(DirtyStateEditorSupport.java:390)
	at org.eclipse.xtext.ui.editor.XtextEditor.initializeDirtyStateSupport(XtextEditor.java:278)
	at org.eclipse.xtext.ui.editor.XtextEditor.doSetInput(XtextEditor.java:260)
	at org.eclipse.xtext.ui.compare.DefaultMergeEditor.doSetInput(DefaultMergeEditor.java:43)
	at org.eclipse.ui.texteditor.AbstractTextEditor$5.run(AbstractTextEditor.java:3154)
	at org.eclipse.jface.operation.ModalContext.runInCurrentThread(ModalContext.java:437)
	at org.eclipse.jface.operation.ModalContext.run(ModalContext.java:353)
	at org.eclipse.ui.internal.WorkbenchWindow$14.run(WorkbenchWindow.java:2195)
	at org.eclipse.swt.custom.BusyIndicator.showWhile(BusyIndicator.java:70)
	at org.eclipse.ui.internal.WorkbenchWindow.run(WorkbenchWindow.java:2191)
	at org.eclipse.ui.texteditor.AbstractTextEditor.internalInit(AbstractTextEditor.java:3172)
	... 65 more

It seems that the Merge Tool uses a StreamContentDocumentProvider instead of an XtextDocumentProvider so that only a plain JFace document object is created.

Reproduce the conflict workflow with an Xtext project, then start by reading XtextEditor, DefaultMergeViewer, DocumentBasedDirtyResource, and the mentioned StreamContentDocumentProvider/XtextDocumentProvider behavior. Confirm that Team → Merge Tool fails during XtextEditor initialization; done means the merge viewer opens the conflicted Xtext document without the null-document exception.