v3.6.12, gitolite mirroring terminology changes

[Sitaram Chamarty]

Hi all,

I've just pushed out a change to the mirroring code and
documentation: "slave"/"slaves" are now "copy"/"copies".

Backward compatibility has been maintained; you do not need to
change either your gitolite.conf, or any scripts you have
written on top, until you are ready to do so. (This in turn
means the word "slave" will still be present in the code, though
only just as much as needed.)

Should you wish to make this change, you first need to migrate
to the latest version (tagged as v3.6.12, so if you want to wait
till the distros pick it up wait for that), and then:

- In the gitolite.conf file, change `option mirror.slaves` to
`option mirror.copies`.

- If you have any scripts that use the `gitolite mirror list
slaves` command, change to `gitolite mirror list copies`.

I've done the usual round of testing, but partly due to Covid,
my "work" environment is no longer on the same infrastructure,
so I can't truly test it "in production", as it were. If you
find any bugs please let me know asap.

sitaram