pickaxe: use textconv for -S counting

pickaxe: use textconv for -S counting

We currently just look at raw blob data when using "-S" to
pickaxe. This is mostly historical, as pickaxe predates the
textconv feature. If the user has bothered to define a
textconv filter, it is more likely that their search string will be
on the textconv output, as that is what they will see in the
diff (and we do not even provide a mechanism for them to
search for binary needles that contain NUL characters).

This patch teaches "-S" to use textconv, just as we
already do for "-G".

Signed-off-by: Jeff King <peff@peff.net>