Tidy up git mergetool's backup file behaviour

Tidy up git mergetool's backup file behaviour

Currently a backup pre-merge file with conflict markers is sometimes
kept with a .orig extenstion and sometimes removed depending on the
particular merge tool used.

This patch makes the handling consistent across all merge tools and
configurable via a new mergetool.keepBackup config variable

Signed-off-by: Charles Bailey <charles@hashpling.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>