git-difftool: Add '--gui' for selecting a GUI tool

git-difftool: Add '--gui' for selecting a GUI tool

Users might prefer to have git-difftool use a different
tool when run from a Git GUI.

This teaches git-difftool to honor 'diff.guitool' when
the '--gui' option is specified.  This allows users to
configure their preferred command-line diff tool in
'diff.tool' and a GUI diff tool in 'diff.guitool'.

Reference: http://article.gmane.org/gmane.comp.version-control.git/133386
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>