Teach Commit View about Encoding

Summary:
If the user specifies a text encoding via the "View Options" dropdown, respect this choice.
Ref Q68.

Test Plan: Play with the Encoding button in the view

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tinloaf, speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25360