watch subcommand

So far this only handles auto-annexing new files that are created inside
the repository while it's running. To make this really useful,
it needs to at least:

- notice deleted files and stage the deletion
  (tricky; there's a race with add..)
- notice renamed files, auto-fix the symlink, and stage the new file location
- periodically auto-commit staged changes
- honor .gitignore, not adding files it excludes

Also nice to have would be:

- Somehow sync remotes, possibly using a push sync like dvcs-autosync
  does, so they are immediately updated.
- Somehow get content that is unavilable. This is problimatic with inotify,
  since we only get an event once the user has tried (and failed) to read
  from the file. Perhaps instead, automatically copy content that is added
  out to remotes, with the goal of all repos eventually getting a copy,
  if df allows.
- Drop files that have not been used lately, or meet some other criteria
  (as long as there's a copy elsewhere).
- Perhaps automatically dropunused files that have been deleted,
  although I cannot see a way to do that, since by the time the inotify
  deletion event arrives, the file is deleted, and we cannot see what
  its symlink pointed to! Alternatievely, perhaps automatically
  do an expensive unused/dropunused cleanup process.

Some of this probably needs the currently stateless threads to maintain
a common state.

GitAnnex.hs

@@ -56,6 +56,7 @@ import qualified Command.AddUrl
 import qualified Command.Map
 import qualified Command.Upgrade
 import qualified Command.Version
+import qualified Command.Watch
 
 cmds :: [Command]
 cmds = concat
@@ -95,6 +96,7 @@ cmds = concat
 	, Command.Map.def
 	, Command.Upgrade.def
 	, Command.Version.def
+	, Command.Watch.def
 	]
 
 options :: [Option]