contrib: add 'git difftool' for launching common merge tools

contrib: add 'git difftool' for launching common merge tools

'git difftool' is a git command that allows you to compare and edit files
between revisions using common merge tools.  'git difftool' does what
'git mergetool' does but its use is for non-merge situations such as
when preparing commits or comparing changes against the index.
It uses the same configuration variables as 'git mergetool' and
provides the same command-line interface as 'git diff'.

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