debian/rules: do not rely on umask to set contrib permissions

debian/rules: do not rely on umask to set contrib permissions

debian-1.7.7-1 (debian/rules, debian/git.postinst: move contrib hooks
from /usr/share/doc/git-core/ into /usr/share/git-core/, 2011-10-09)
removed the following code:

	find '$(GIT)'/usr/share/doc/git/contrib -type f | xargs chmod 0644
	find '$(GIT)'/usr/share/doc/git/contrib -type d | xargs chmod 0755

But making such a change without advertising it in debian/changelog is
not so nice, and without those two lines, the permissions of files
under contrib/ vary based on umask in the build environment.

Later it probably makes sense to tweak the find expression so we can
keep executable files executable while still explicitly setting
permissions.  For now let's just restore the pre-1.7.7 behavior.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>