Tell DOMDocument that our diff is UTF-8 encoded.

Otherwise non-ASCII text in diff is garbled. It's decoded as ISO-8859-1
as DOMDocument input, then encoded as UTF-8 as DOMDocument output.

This is hacky, but the whole function is already hacky enough.

Change-Id: Ic9c437bf1c32eea70924836080ba5591cffd49e7
Merge "Tell DOMDocument that our diff is UTF-8 encoded."