How to check what files have been customised in /etc?

[Chris Green]

I try as far as possible to keep my system 'standard' with as much as
possible of the custmisation being in /home where I use mercurial to
keep track of changes (and which is preserved when OS is upgraded).

However there are inevitably a few global customisations in /etc and
I'd like to track them as well if possible.  I can configure ways to
track them now but can anyone suggest a way to find all the changes I
have done over the years in /etc?  I don't think there are all that
many of them but I know there are some and there are likely to be some
I can't remember how I did.

A typical example is that I have changed /etc/systemd/system/anacron.timer
to change the times when anacron does its checks.  I wanted to change
this recently and it took me ages to find out what to change.  I want
to find and start tracking changes like this.

-- 
Chris Green