Git: Also use the latest settings setup approach

Change-Id: I34a210575d02d18927c1e0f6d8ea6cb9924c563d
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/git/gitsettings.h

@@ -46,10 +46,12 @@ public:
     Utils::FilePath gitExecutable(bool *ok = nullptr, QString *errorMessage = nullptr) const;
 };
 
+GitSettings &settings();
+
 class GitSettingsPage final : public Core::IOptionsPage
 {
 public:
-    explicit GitSettingsPage(GitSettings *settings);
+    GitSettingsPage();
 };
 
 } // Git::Internal