Add live log updates in jj-fzf via --watch mode

[tim-janik]

Originally posted by @thoughtpolice in #5121:

I believe jj-fzf also provides support for automatic updates, but I don't use it.

At the moment jj-fzf reloads the jj log when F5 is pressed.

A recurring question from JJ users is howto get live updates of a jj log.

We should add a --watch mode that uses e.g. entr to peek at .git / .jj files (or fallback to stat + sleep).
When changes are detected, trigger a reload event in fzf.
This can be done in newer fzf versions via the fzf --listen option (in combination with a $FZF_API_KEY).