Git commit notifications via post-receive hook

Git commit notifications via post-receive hook

Now that we moved our Git repositories to SourceForge, we don't need to
maintain local clones for generating commit notifications anymore, as
SourceForge provides shell access to the repositories.  Instead, we now
run git-notify as a post-receive hook on the SourceForge server.
Actually, we use a wrapper which executes git-notify with the desired
options and which makes it easy to add other post-receive hooks in the
future.