mark contributed hooks executable

mark contributed hooks executable

The docs in contrib/hooks/pre-auto-gc-battery suggest:

	For example, if the hook is stored in
	/usr/share/git-core/contrib/hooks/pre-auto-gc-battery:

	chmod a+x pre-auto-gc-battery
	cd /path/to/your/repository.git
	ln -sf /usr/share/git-core/contrib/hooks/pre-auto-gc-battery \
	     hooks/pre-auto-gc

Unfortunately on multi-user systems most users do not have write
access to /usr.  Better to mark the sample hooks executable in
the first place so users do not have to tweak their permissions to
use them by symlinking into .git/hooks/.

Reported-by: Olivier Berger <olivier.berger@it-sudparis.eu>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>