diff: allow turning on textconv explicitly for plumbing

diff: allow turning on textconv explicitly for plumbing

Some history viewers use the diff plumbing to generate diffs
rather than going through the "git diff" porcelain.
Currently, there is no way for them to specify that they
would like to see the text-converted version of the diff.

This patch adds a "--textconv" option to allow such a
plumbing user to allow text conversion.  The user can then
tell the viewer whether or not they would like text
conversion enabled.

While it may be tempting add a configuration option rather
than requiring each plumbing user to be configured to pass
--textconv, that is somewhat dangerous. Text-converted diffs
generally cannot be applied directly, so each plumbing user
should "opt in" to generating such a diff, either by
explicit request of the user or by confirming that their
output will not be fed to patch.

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