Add a safeguard against multiple patches with the same version

Summary:
I accidentally added two "104" patches. This actually works OK for the most part
but is fundamentally bad and wrong.

Merge the patches (installs applied both as "104", so we can't move one to
"105") and add a safeguard.

Test Plan: Ran upgrade_schema.php with two "104" patches, got error'd. Ran
without, got successs.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1614