set up watchdog to notify AutoCompleter when files are changed externally

[jbellis]

Fixes #1533

[fry69]

@jbellis Does pressing Enter work for you to refresh the autocompleter (as @paul-gauthier mentioned in the related issue) and is this an acceptable solution for you?

[jbellis]

For me personally, sure. I thought this is the kind of quality-of-life improvement that would be nice to have as aider matures and broadens its userbase, but if I am wrong feel free to close it out.

[jbellis]

Update: I find that taking a few seconds wondering why completion isn't working, then C-a C-k to preserve my partial command, enter, C-y to edit again, is distracting and I would definitely prefer to have it refresh automatically in the background.

[CLAassistant]

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