git: update hook, fix post-receive-email

git: update hook, fix post-receive-email

 Add an update hook that prevents merge commits on master.

 Fix the post-receive-email script to work properly with merge commits --
 don't try to report pushes of merges with one email per commit.

 (Since we deny non-fast-forwards, we pretty much need to allow merges on
 other branches, or give up keeping them in the main repo.)