I just wrote a very rough and quick walkthrough how to get that tool running:
https://www.mediawiki.org/wiki/Arcanist
My first impression is very good. The UI is very nice (it guides you
when it needs to, it just does the job if all is fine).
The user's guide is unfortunately poor.
I don't know yet how to avoid this warning:
This diff is for the 'E3Experiments' project but the working copy belongs to the
'' project.
I see that arc can be also installed as the git pre-receive hook,
but it needs some project configuration for that. Interesting.
Anyway, I managed to download one change:
$ git branch -vv
* arcpatch-D3 be7cfd9 Merge branch 'master' of
ssh://gerrit.wikimedia.org:29418/mediawiki/extensions/E3Experiments into munaf/pef2
master e40fce0 [origin/master] Rename events.js -> communityClicks.js
//Saper