Handle encoding if git tells us it's still not UTF-8

Summary:
Even though --encoding is passed to the command, git still fails
in some cases to correctly convert the output. Attempt the conversion
ourselves if it's non UTF-8.

Test Plan: Reparsed message in a repository with ISO-8859-1 encoded commit messages.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T452

Differential Revision: https://secure.phabricator.com/D2888