[users at bb.net] A few notes about gitlab support

[Dan Kegel]

I must say, I'm enjoying adding my little tweaks to gitlab support,
https://github.com/buildbot/buildbot/pull/4108
Having done a hacky job of it in the 0.8 era, it's nice to do it right
this time.

Three notes:
1)
While I was testing I came across this:
https://irclogs.jackgrigg.com/irc.freenode.net/buildbot/2018-04-10
> there is again bug in gitlab reporter :(
>
> 2018-04-10 16:19:34+0000 [-] Could not send status "running" for gitgit.dovecot.net:dovecot/core.git at 5441373137620aa8094ceafd9e698d430f5ed0d0: Cannot transition status via :run from :running (Reason(s): Status cannot transition via "run")
This happened to me... when I screwed up my master.cfg
such that changesets were added to two different builders
at once, causing what looked like duplicate status reports
to gitlab, which evidently objected to the duplication.
The problem went away once I added a project field to my ChangeFilter :-)

2)
Oddly, the project field was blank in the gitlab webhook;
I guess it used to be in the outer header, or something.
I had to change the hook to get it from the payload.

3)
A few things I worry about a bit:
- case insensitivity in project names (I only use/test lowercase)
- whether it's bad to ignore group / namespace ownership (I do,
  and am waiting for someone to tell me I'm an idjit)
- nested projects, e.g. with another / in the name (I haven't tried it)

- Dan