Fetch only the recent commits when cloning a remote Git repository for mirroring

[trustin]

Currently, Central Dogma clones the entire repository when mirroring a remote Git repository. We don't need to fetch the entire graph but only the latest tree. We could specify --depth option so we don't fetch the Git objects that are not used actually.

Related API:

• FetchCommand.setDepth(int)
  • https://download.eclipse.org/jgit/site/6.4.0.202211300538-r/apidocs/org/eclipse/jgit/api/FetchCommand.html#setDepth(int)

Also, we should:

• ensure old commits are also pruned when the remote Git repo adds a new commit or force-pushes; and
• write a test

[trustin]

Closed via #808 Thanks a lot, @thachlp! 🙇

[thachlp]

Thank you for your support @trustin