Repository watcher needs to be fixed

[rowanj]

There are several outstanding issues regarding repository watcher, and even a couple of pull requests; but there are underlying issues that need a Better Solution, due to poor performance and broken behaviour.

• Watch the work dir (a bug from earlier this year means that it's only watching the index)
• Watch the .git and work tree individually (to handle submodules)
• Check the status of individual files per event info instead of enumerating the repository
• Split repository and file changed events
• Don't refresh the index unless the index has changed
• Don't refresh the history unless the refs change

[rjmunro]

Can we add deferred / throttled updates? I.e. wait 1 second from an event happening before we update the UI, in case something else happens in that time. #229

[rowanj]

The new watcher code doesn't enumerate the repository status like the old one did; so it's much faster, especially under load.

The FSEventStreamCreate call itself has a latency parameter that should coalesce updates within a specific period and effectively apply the same throttling; it's currently set to 1.0s.

Is the throttling necessary with the testing builds that include this change?
https://github.com/rowanj/gitx/releases/tag/builds/testing/2

[rjmunro]

I tried rebasing on the command line on a large repository while watching with this new build - it didn't suffer the slowdown I get with the current live version, but at the end, it didn't reload the new position - the space where the history tree should be was blank until I clicked on the branch name again.

The other problem I had with this build is nothing to do with the file watcher, so I made a separate issue: #247. Also the build thought it was an old version that needed updating, but that presumably won't happen when it becomes a release...