<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def git_ls_remote(self, uri, ref):
 """Determine the latest commit id for a given ref. Args: uri (string):
 git URI ref (string):
 git ref Returns: str: A commit id """

        logger.debug("Invoking git to retrieve commit id for repo %s...", uri)
        lsremote_output = subprocess.check_output(['git',
                                                   'ls-remote',
                                                   uri,
                                                   ref])
        if b"\t" in lsremote_output:
            commit_id = lsremote_output.split(b"\t")[0]
            logger.debug("Matching commit id found: %s", commit_id)
            return commit_id
        else:
            raise ValueError("Ref \"%s\" not found for repo %s." % (ref, uri))