Finish "git annex watch" command, which runs, in the background, watching via
inotify for changes, and automatically annexing new files, etc.

There is a `watch` branch in git that adds such a command. To make this
really useful, it needs to:

- on startup, add any files that have appeared since last run **done**
- on startup, fix the symlinks for any renamed links **done**
- on startup, stage any files that have been deleted since last run
  (seems to require a `git commit -a` on startup, or at least a
  `git add --update`, which will notice deleted files) **done**
- notice new files, and git annex add **done**
- notice renamed files, auto-fix the symlink, and stage the new file location
  **done**
- handle cases where directories are moved outside the repo, and stop
  watching them **done**
- when a whole directory is deleted or moved, stage removal of its
  contents from the index **done**
- notice deleted files and stage the deletion
  (tricky; there's a race with add since it replaces the file with a symlink..)
  **done**
- periodically auto-commit staged changes (avoid autocommitting when
  lots of changes are coming in)
- tunable delays before adding new files, etc
- Coleasce related add/rm events. See commit
  cbdaccd44aa8f0ca30afba23fc06dd244c242075 for some details of the problems
  with doing this.
- don't annex `.gitignore` and `.gitattributes` files, but do auto-stage
  changes to them
- configurable option to only annex files meeting certian size or
  filename criteria
- honor .gitignore, not adding files it excludes (difficult, probably
  needs my own .gitignore parser to avoid excessive running of git commands
  to check for ignored files)
- Possibly, when a directory is moved out of the annex location,
  unannex its contents.
- Gracefully handle when the default limit of 8192 inotified directories
  is exceeded. This can be tuned by root, so help the user fix it.
- Support OSes other than Linux; it only uses inotify currently.
  OSX and FreeBSD use the same mechanism, and there is a Haskell interface
  for it,