refactor get_textconv to not require diff_filespec

This function actually does two things:

1. Load the userdiff driver for the filespec.

2. Decide whether the driver has a textconv component, and
initialize the textconv cache if applicable.

Only part (1) requires the filespec object, and some callers
may not have a filespec at all. So let's split them it into
two functions, and put part (2) with the userdiff code,
which is a better fit.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>