Get s3uri from config

Git lfs allows for an endpoint url to be set in the git config. This tool currently just assumes the endpoint is the remote url.
The s3uri can be gathered from the values "lfs.pushurl" and similar, as found in https://github.com/git-lfs/git-lfs/blob/main/docs/man/git-lfs-config.adoc.

I suggest that the s3uri which is derived in lfs.py is taken from the git config as per the keys described in the lfs documentation. This allows for a seperate lfs endpoint. For example, github can be used for regular git data, and s3 for lfs data.