Do not run the callback when watchfiles starts up

[frank-lenormand]

Description

Hi,

It seems that watchfiles runs the given command upon starting up.

This is unexpected to me, as I use the tool to run a shell command when the watched files are modified.

Example Code

$ mkdir /tmp/t
$ cd /tmp/t
$ watchfiles 'echo OK' .

### Watchfiles Output

```Text
[11:20:56] watchfiles v0.19.0 👀  path="/tmp/t" target="echo OK" (command) filter=DefaultFilter...
OK

### Operating System & Architecture

Linux-5.15.0-69-generic-x86_64-with-Ubuntu-20.04-focal
#76~20.04.1-Ubuntu SMP Mon Mar 20 15:54:19 UTC 2023

### Environment

_No response_

### Python & Watchfiles Version

python: 3.8.10 (default, Mar 13 2023, 10:26:41)  [GCC 9.4.0], watchfiles: 0.19.0

### Rust & Cargo Version

_No response_

[fritzo]

It would be great if this were controllable, or if EXISTS_ON_INIT were a separate event type users like @frank-lenormand could filter out. OTOH it's helpful in some applications to get all files that exist and also get future changes to them, as it's tricky to orchestrate the directory walking and then get change events.

[frank-lenormand]

For reference, here is the list of events that inotifywatch supports:

https://github.com/inotify-tools/inotify-tools/blob/master/man/inotifywatch.1.in#L182