R1:bbf4ce79e341

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…