git-grep-filter - truncate lines to terminal column width

We make use of the Haskell C FFI and the `ioctl` function from <sys/ioctl.h>
to obtain the number of columns from `struct winsize`. If `ioctl` returns -1,
we default to using 80 columns.

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>