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?

首先阅读 issue 1119 并跟踪 GlobalSettings 和 Repository 类型，尤其要了解注册如何比 Repository 实例存活得更久。明确在 repository 保持存活期间，保护或锁应保证什么，以及文档是否应描述这些生命周期约束；当预期行为和范围达成一致时即视为完成。