Add In-Memory Repository Support

I am building an application that needs to interact with git (clone repo, create repo, push, etc.) but does not have direct access to a disk. I would like to be able to work with a git repo entirely in-memory.

As an aside, I would also like to be able to work with a git repository that is written out to a non-file datastore such as a database.

In general, it would be nice if the logic in Repository was abstracted out away from the disk. It appears that IRepository is not tightly coupled with disk (assuming all of the various methods that take a path are referring to a relative path). However, there appears to be quite a bit of git specific logic bound up in the Repository implementation which makes it difficult to a novice contributor to implement a custom datastore engine (e.g., in-memory, database, etc.).

Researching this topic very briefly, it appears that the underlying libgit2 supports non-filesystem repositories, so I don't believe this is a technical limitation with libgit2.

首先检查 IRepository 契约和 Repository 实现，然后验证 libgit2 为非文件系统仓库提供了哪些支持。定义内存中数据存储和其他非文件数据存储所需的抽象，并通过 clone、create 和 push 等仓库操作能够在不直接访问磁盘的情况下运行来证明完成。