Provide transaction-based edits of repository encoding

Summary: Adds support for the "encoding" field to the new transactional interface.

Test Plan:
{F44189}

{F44190}

Some of the encodings in the second screen are from testing, and can no longer be set.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6035