Git filters are not applied on working file for plain text diff

[fcollonval]

When requesting the content of a working file for the plain text diff, git filters that could apply to those files are ignored:

jupyterlab-git/jupyterlab_git/git.py

Lines 928 to 934 in 576c444

	def get_content(self, filename, top_repo_path):
	"""
	Get the file content of filename.
	"""
	relative_repo = os.path.relpath(top_repo_path, self.root_dir)
	model = self.contents_manager.get(path=os.path.join(relative_repo, filename))
	return model['content']

We could take advantage of the following code in ndime:
https://github.com/jupyter/nbdime/blob/6259cd0936eecae3e0fbd4301ecc55f4a5e19bc2/nbdime/vcs/git/filter_integration.py