Git: Store singleton instance in GitClient

On many cases, GitPlugin is not required at all, and is only used as
a proxy for GitClient.

Change-Id: I246012658ab3e8c7a12f1a459b1b1748ff358e0b
Reviewed-by: hjk <hjk@qt.io>

src/plugins/git/gitclient.h

@@ -139,6 +139,7 @@ public:
     };
 
     explicit GitClient(GitSettings *settings);
+    static GitClient *instance();
 
     Utils::FilePath vcsBinary() const override;
     unsigned gitVersion(QString *errorMessage = nullptr) const;