Use Git's encoding flag instead of MBString

Summary:
0d5b0f21ad2bc03988b007b017e44d4210ab9645 added string conversion but MBString always needs an argument for endcoding.

It looks like we can get away with doing this in git instead, with the --encoding='UTF-8' flag. Then we should be safe to remove the test for output type, and stay UTF-8 safe.

Test Plan:
Run updaters with change. Verify commits are updated.

Reviewers: epriestley

CC:

Differential Revision: 1108