git-mergetool.sh mostly uses 8 space tabs and 4 spaces per indent. This
change corrects this in a part of the file affect by a later commit in
this patch series. diff -w considers this change is to be a null change.
Signed-off-by: Charles Bailey <charles@hashpling.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>