Provide a username and email when running git merge --squash

Summary:
Ref T182. This command should never actually generate a commit because --squash prevents that, but git seems to sometimes hit a check for username/email configuration (maybe when merging a non-fastforward?).

Give it some dummy values to placate it. This command shouldn't commit anything so these values should never actually be used.

Test Plan: Landed rGITTESTd8c8643cb02bbe60048c6c206afc2940c760a77e.

Reviewers: chad, Mnkras

Reviewed By: Mnkras

Maniphest Tasks: T182

Differential Revision: https://secure.phabricator.com/D14347