POC: Rework update file via git fast-import

[lunny]

This PR introduced a new implementation for online files changes.

The previous implementation will clone the repository and use git index. This will take much time if the repository is big.
The new implementation is based on git fast-import which can run in bare repository and without checkout.

Pros

• High performance when importing many files

Cons

• Hooks will not be triggered directly by git
• No git transaction guaranteed. i.e. Gitea needs the hooks to sync branches names to database