Shorten extremely long credential names when migrating them

Summary: Fixes T4183. If you have too many repositories sharing the same credential and MySQL is in strict mode, we'll fail a query when trying to write a credential with a name longer than 255 characters. Instead, shorten the variable-length part to 128 characters.

Test Plan: Wiped credentials column and successfully re-ran migration with `storage upgrade --force --apply phabricator:20131121.repocredentials.2.mig.php`

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4183

Differential Revision: https://secure.phabricator.com/D7677