Fix improper viewer for Git SSH cluster workflows

Summary: Ref T10751. These workflows have separate getUser() and getViewer() for weird legacy reasons. getUser() is correct.

Test Plan:

• Did a Git SSH push, verified that "Last Writer" reflected the proper user in the "Storage" UI in repository management.
• Grepped for other callsites, double-checked that they used correct users.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10751

Differential Revision: https://secure.phabricator.com/D15893