Documentation/technical/api-fswatch.txt: start with outline

git operations are slow on repositories with lots of files, and lots
of tiny filesystem calls like lstat(), getdents(), open() are
reposible for this.  On the linux-2.6 repository, for instance, the
numbers for "git status" look like this:

  top syscalls sorted     top syscalls sorted
  by acc. time            by number
  ----------------------------------------------
  0.401906 40950 lstat    0.401906 40950 lstat
  0.190484 5343 getdents  0.150055 5374 open
  0.150055 5374 open      0.190484 5343 getdents
  0.074843 2806 close     0.074843 2806 close
  0.003216 157 read       0.003216 157 read

To solve this problem, we propose to build a daemon which will watch
the filesystem using inotify and report batched up events over a UNIX
socket.  Since inotify is Linux-only, we have to leave open the
possibility of writing similar daemons for other platforms.
Everything will continue to work as before if there is no helper
present.

The fswatch API introduces a generic way for git.git to request for
filesystem changes.  Different helpers (like the inotify daemon on
Linux) will be plugged into this API on different platforms.  It falls
back to using the filesystem calls.

The daemon will start up with the very first operation done on the git
repository, and will die after a specified period of repository
inactivity.  It is going to be a per-repo daemon and will write to a
socket in the repository: access control is managed by filesystem
permissions.

This design is inspired by the credential helper design.
[WIP] fswatch stuff