Enable passing through encoding to perforce tool.

I believe perforce is utf8 by default, but some users want other encodings (such
as utf16, shiftjis, etc). This change enables passing through the encoding to
the perforce tool.

I've added a unit test, which at the moment isn't as complete as I'd like
because public.perforce.com doesn't have "unicode" turned on. It at least tests
that we pass the parameter through to the underlying API/p4 binary, and fails
with a useful message should they update the server.

Ran perforce tests.