mergetool--lib: set IFS for difftool and mergetool

git-sh-setup sets IFS but it is not used by git-difftool--helper.
Set IFS in git-mergetool--lib so that the mergetool scriptlets,
difftool, and mergetool do not need to do so.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-mergetool--lib.sh

@@ -2,6 +2,9 @@
 
 : ${MERGE_TOOLS_DIR=$(git --exec-path)/mergetools}
 
+IFS='
+'
+
 mode_ok () {
 	if diff_mode
 	then