DiffEditorDocument: Assign mimetype

It helps when savingAs vcs diffs for the first time.

Change-Id: I53b1c2c89641ee6b8106fb574c050ead60aa8da4
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/diffeditor/diffeditordocument.cpp

@@ -46,6 +46,7 @@ DiffEditorDocument::DiffEditorDocument() :
     m_controller(new DiffEditorController(this))
 {
     setId(Constants::DIFF_EDITOR_ID);
+    setMimeType(QLatin1String(Constants::DIFF_EDITOR_MIMETYPE));
     setTemporary(true);
 }