HackStudio: Fix GitWidget issues with switching between projects

GitWidget no longer crashes upon trying to create a repository for a new
project, and it correctly updates after opening a different project. A
new method, change_repo, has been added to make this work, which changes
m_repo_root and resets most of the widget's state in order to make this
work.

Userland/DevTools/HackStudio/Git/GitWidget.cpp

@@ -164,4 +164,12 @@ void GitWidget::show_diff(const LexicalPath& file_path)
     VERIFY(original_content.has_value() && diff.has_value());
     m_view_diff_callback(original_content.value(), diff.value());
 }
+
+void GitWidget::change_repo(LexicalPath const& repo_root)
+{
+    m_repo_root = repo_root;
+    m_git_repo = nullptr;
+    m_unstaged_files->set_model(nullptr);
+    m_staged_files->set_model(nullptr);
+}
 }

Userland/DevTools/HackStudio/Git/GitWidget.h

@@ -24,6 +24,7 @@ public:
     void refresh();
     void set_view_diff_callback(ViewDiffCallback callback);
     bool initialized() const { return !m_git_repo.is_null(); };
+    void change_repo(LexicalPath const& repo_root);
 
 private:
     explicit GitWidget(const LexicalPath& repo_root);

Userland/DevTools/HackStudio/HackStudioWidget.cpp

@@ -202,6 +202,10 @@ void HackStudioWidget::open_project(const String& root_path)
         m_project_tree_view->set_model(m_project->model());
         m_project_tree_view->update();
     }
+    if (m_git_widget) {
+        m_git_widget->change_repo(LexicalPath(root_path));
+        m_git_widget->refresh();
+    }
     if (Debugger::is_initialized()) {
         auto& debugger = Debugger::the();
         debugger.reset_breakpoints();