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

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>