Advices to imlement update hook

Hi !

I'd like to restrict access to one of my repos: the project tracked by
it has several directories: a/ b/ c/ and I'd like to allow only
modifications happen in b/ directory.

I'm planning to use git hooks, not sure which one I should use: if the
check should happen at the commit or push time or both.

For now I'm going to use the update hook, so it will happen when pushing.

Now the question is: what is the best way to do this ?

I'm planning to use something equivalent to "git-diff-tree oldref
newref | { grep -v b || exit 1; }" but doesn't look like the best git
way.

BTW, from the git-diff-tree manpage:

	<path>...
	    If provided, the results are limited to a subset of
	    files matching one of these prefix strings. i.e., file
	    matches /^<pattern1>|<pattern2>|.../ Note that this
	    parameter does not provide any wildcard or regexp
	    features.

What does it mean exactly ? what does 'pattern' word mean if wildcard
or regexp is not supported ?
I also tried:

   git diff-tree <oldref> <newref> -- ^b || exit 1

but it doesn't work.

Could anybody give me some advices ?

Thanks
-- 
Francis