Makefile: fix permissions of mergetools/ checked out with permissive umask

Ever since mergetool--lib was split into multiple files in v1.7.7-rc0~3^2~1 (2011-08-18), the Makefile takes care to reset umask and use tar --no-owner when installing merge tool definitions to $(gitexecdir)/mergetools/. Unfortunately it does not take into account the possibility that the permission bits of the files being copied might already be wrong. Rather than fixing the "tar" incantation and making it even more complicated, let's just use the "install" utility. This only means losing the ability to install executables and subdirectories of mergetools/, which wasn't used. Noticed by installing from a copy of git checked out with umask 002. Compare v1.6.0.3~81^2 (Fix permission bits on sources checked out with an overtight umask, 2008-08-21). Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>