(stable) Fix a missing getSSHUser() callsite

Summary:
See https://discourse.phabricator-community.org/t/after-upgrade-git-lfs-push-ends-up-in-call-to-undefined-method-on-diffusion-git-lfs-authenticate-workflow/1047/1.

I renamed this method in D18912 but missed this callsite since the workflow doesn't live alongside the other ones.

Test Plan: Ran git push in an LFS repository over SSH. Before: fatal; after: clean push.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18977