Pass --no-textconv when running git diff internally

Pass --no-textconv when running git diff internally

Seems that --no-ext-diff and -c diff.external= are not enough to disable
external diff command when gitattributes textconv specifies it.

I'm pretty sure that --no-ext-diff and -c diff.external= are not both
needed, but not 100%. Something about -G may need the latter to fully
disable diffs in some cases. So kept that part as it was.

Sponsored-by: Dartmouth College's Datalad project