Avoid checking out worktree for --git

[NobodyXu]

• Support symlink handling in git::Repository::get_head_commit_entry_data_by_path
• Support listing dir and checking whether a path is a dir in git::Repository
• Use git::Repository::shallow_clone_bare in resolve.rs#L374 and modify load_manifest_from_workspace to support use git::Repository instead of worktree.

[NobodyXu]

Hmmm it seems like that if we use shallow_clone_bare when dealiing when user provided --git, then we won't be able to handle submodule...

And load_manifest_from_workspace would have to be way more complicated with a trait for virtual filesystem.

Given that it is unlikely for any repository to be as large as crates.io-registry, I think it's better left as-is.