git-p4: add diff/merge properties to .gitattributes for GitLFS files
The `git lfs track` command generates a .gitattributes file with diff
and merge properties [1]. Set the same .gitattributes format for files
tracked with GitLFS in git-p4.
Signed-off-by: Lars Schneider <larsxschneider@gmail.com> Reviewed-by: Luke Diamand <luke@diamand.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>