Protect repository instances from domain wide GlobalSettings changes

As seen in https://github.com/libgit2/libgit2sharp/issues/1119

• Paybe should we also improve the documentation of this type. Indeed, this type holds registrations that exceeds the lifespan of a Repository.I feel that pretty strange side-effects may happen would we blindly deregister while some repos are still alive and leveraging those.
• Perhaps some kind of locking of global values while any Repository objects are not yet disposed could provide some value?

首先閱讀 issue 1119 並追蹤 GlobalSettings 和 Repository 型別，尤其要了解註冊如何比 Repository 執行個體存活得更久。明確在 repository 持續存活期間，保護或鎖定應保證什麼，以及文件是否應描述這些生命週期限制；當預期行為和範圍達成共識時即視為完成。