RevCommit: Better support invalid encoding headers

RevCommit: Better support invalid encoding headers

With this support we no longer need the 'utf-8' alias. UTF-8 will be
automatically tried when the encoding header is not recognized and used
if the character sequence cleanly decodes as UTF-8.

Modernize some of the references to use StandardCharsets.

Change-Id: I4c0c88750475560e1f2263180c4a98eb8febeca0