Move git specific stuff out of diff editor plugin

Move it to the git plugin.

Change-Id: I8151573ed50df70776f7ebf0475dd41fb84fae83
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/diffeditor/descriptionwidgetwatcher.cpp

@@ -0,0 +1,78 @@
+/****************************************************************************
+**
+** Copyright (C) 2018 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#include "descriptionwidgetwatcher.h"
+#include "diffeditor.h"
+#include "diffeditorcontroller.h"
+
+#include <coreplugin/editormanager/documentmodel.h>
+#include <coreplugin/editormanager/editormanager.h>
+#include <texteditor/texteditor.h>
+
+using namespace Core;
+
+namespace DiffEditor {
+
+DescriptionWidgetWatcher::DescriptionWidgetWatcher(DiffEditorController *controller)
+    : QObject(controller), m_document(controller->document())
+{
+    const QList<IEditor *> editors = DocumentModel::editorsForDocument(controller->document());
+    for (auto *editor : editors) {
+        if (TextEditor::TextEditorWidget *widget = descriptionWidget(editor))
+            m_widgets.append(widget);
+    }
+
+    connect(EditorManager::instance(), &EditorManager::editorOpened,
+            [this](IEditor *editor) {
+        if (TextEditor::TextEditorWidget *widget = descriptionWidget(editor)) {
+            m_widgets.append(widget);
+            emit descriptionWidgetAdded(widget);
+        }
+    });
+
+    connect(EditorManager::instance(), &EditorManager::editorAboutToClose,
+            [this](IEditor *editor) {
+        if (TextEditor::TextEditorWidget *widget = descriptionWidget(editor)) {
+            emit descriptionWidgetRemoved(widget);
+            m_widgets.removeAll(widget);
+        }
+    });
+}
+
+QList<TextEditor::TextEditorWidget *> DescriptionWidgetWatcher::descriptionWidgets() const
+{
+    return m_widgets;
+}
+
+TextEditor::TextEditorWidget *DescriptionWidgetWatcher::descriptionWidget(Core::IEditor *editor) const
+{
+    if (Internal::DiffEditor *diffEditor = qobject_cast<Internal::DiffEditor *>(editor)) {
+        if (diffEditor->document() == m_document)
+            return diffEditor->descriptionWidget();
+    }
+    return nullptr;
+}
+
+} // namespace DiffEditor