Hacking on a bus to Canberra for [LCA2013](https://lca2013.linux.org.au/),
I made the webapp's UI for pausing syncing to a repository also work for
the local repository. This pauses the watcher thread. (There's also an
annex.autocommit config setting for this.)

Ironically, this didn't turn out to the use the thread manager I built
yesterday. I am not sure that a ThreadKilled exception would never be
masked in the watcher thread. (There is some overly broad exception
handling in git-annex that dates to back before I quite understood haskell
exceptions.)
