Small fix for git versioner

Since git ls-remote may return several lines, I think there is a chance that the wrong version is selected in the git versioner’s repo_version action.

For instance, on my local ducttape repository, the command `git ls-remote . HEAD` returns commits for both `HEAD` and `refs/remotes/origin/HEAD`.

While the current version seems to work anyway, I’ve added a grep statement to the line in question to make sure only one line is returned.