optimize git status method with first commit check

[dualc]

Problem

The status method always uses empty tree SHA comparison even for repositories with existing commits, which is unnecessary.
this command git diff --numstat -z --cached 4b825dc642cb6eb9a060e54bf8d69288fbee4904 is very slow when git proejct is large and is in filesystem like NFS

https://github.com/jupyterlab/jupyterlab-git/blob/861374ffa6b2edc382d37eec6185eca3b1695787/jupyterlab_git/git.py#L489C15-L489C21

Solution

Add _is_first_commit helper method to detect empty repositories and only use empty tree SHA when it's the first commit.