contrib/difftool: change trap condition from SIGINT to INT

contrib/difftool: change trap condition from SIGINT to INT

git-difftool worked for me on an up-to-date Gentoo Linux at home, but
didn't work on a somewhat older Ubuntu Linux 7.10 at work and failed
with the following error, where 'Makefile' was locally modified:

    trap: 244: SIGINT: bad trap
    external diff died, stopping at Makefile.

In 'man 1p trap' there is written:

    "The condition can be EXIT, 0 (equivalent to EXIT), or a signal
    specified using a symbolic name, without the SIG prefix, [...]"

    "Implementations may permit names with the SIG prefix or ignore case
    in signal names as an extension."

So now we do it the POSIX compliant way instead of using an extension.

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>