Support shallow fetch

Hi,

I integrate this project with Bazel. Bazel will fetch third-party git repos from S3 during the build process. It always pops up a message, something like: `shallow fetch failed, fetching full history`.

From the Bazel docs:
> Bazel will first try to perform a shallow fetch of only the specified commit. If that fails (usually due to missing server support), it will fall back to a full fetch of the repository.

I believe we will have better efficiency if we have the shallow fetch feature. Thanks for your support.

Reference:
- Git option `depth`
https://git-scm.com/docs/git-fetch#Documentation/git-fetch.txt---depthltdepthgt
- Bazel git repository rules
https://bazel.build/rules/lib/repo/git