difftool: provide basename to external tools

difftool: provide basename to external tools

Currently, only configured diff helpers get the basename of the file
being compared. Tools specified with "git difftool -x" only get the
names of temporary files for the different versions.

Export BASE so that an external tool can read the name from the
environment. Rather than using a third argument, this avoids breaking
existing scripts which may somewhat carelessly be using "$@" rather than
"$1" "$2".

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>