update with some gotchas about how git calls hooks

doc/tips/setup_a_public_repository_on_a_web_site.mdwn

@@ -20,12 +20,34 @@ Here's how I set it up. --[[Joey]]
 7. Instruct advanced users to clone a http url that ends with the "/.git/"
    directory. For example, for downloads.kitenet.net, the clone url
    is `https://downloads.kitenet.net/.git/`
-8. Set up a git `post-receive` hook that runs `git annex merge`, and
+8. Set up a git `post-receive` hook to update the repository's working tree
-   the repository's working tree will automatically be updated when
+   when changes are pushed to it. See below for details.
-   you run `git annex sync` in a clone that can push to the repository.  
-   (Needs git-annex version 4.20130703 or newer; older versions
-   can use `git annex sync` in the post-receive hook instead.)
 
 When users clone over http, and run git-annex, it will
 automatically learn all about your repository and be able to download files
 right out of it, also using http. 
+
+## post-receive hook
+
+If you have git-annex 4.20130703, the post-receive hook mentioned above
+in step 8 just needs to run `git annex merge`.
+
+With older versions of git-annex, you can instead use `git annex sync`.
+
+There are two gotchas with some versions of git to be aware of when writing
+this post-receive hook.
+
+1. The hook may be run with the current directory set to the `.git`
+   directory, and not the top of your work tree. So you need to `cd ..` or
+   similar in the hook.
+2. `GIT_DIR` may be set to `.`, which will not be right after changing
+   directory. So you will probably want to unset it.
+
+Here's a post-receive hook that takes these problems into account:
+
+<pre>
+#!/bin/sh
+unset GIT_DIR
+cd ..
+git annex merge
+</pre>