git mergetool listed some candidates for mergetools twice, depending on
the environment.
This slightly changes the behavior when both KDE_FULL_SESSION and
GNOME_DESKTOP_SESSION_ID are set at the same time; in such a case
meld is used in favor of kdiff3 (the old code favored kdiff3 in such a
case), but it should not matter in practice.
Signed-off-by: Johannes Gilger <heipei@hackvalue.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>