This patch adds a fill_textconv wrapper, which centralizes
some minor logic like error checking and handling the case
of no-textconv.

In addition to dropping the number of lines, this will make
it easier in future patches to handle multiple types of
textconv.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>