Support shallow fetch

[louiswen7]

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

[massi-ang]

Hi @louiswen7 , I did some research and implementing a solution which is equivalent to the shallow fetch is not feasible with the current design. It could be possible to implement shallow fetch for the latest commit only, but I have not found any way to perform an unshallow on that repo, which could cause more issues than it solves.