test building a gerrit patch using tinbuild2

I have pushed improvement to tinbuild2 to allow to automotate the test
building of gerrit patcheset.

prereq:

+ having tinbuild2  (clone contrib/buildbot)
+ having configured it. (see README.tinbuild2 (*)). I suggest defining
a profile 'gerrit' with a ~/.tinbuild/autogen/gerrit.autogen such as
the build only need the core repo:
something with
--with-help=no
--with-myspell-dicts=no
--disable-binfilter


+ add an alias 'gerrit' in your .ssh/config like

Host gerrit
User <username_in_gerrit>
Port 29418
Hostname gerrit.libreoffice.org
IdentityFile <path_to_the_private_key_associated_to_the_public_key_you_regestered_to_gerrit>

(Yes right know that alias is hard-coded in tindbuild2. it would be
trivial to make that a config option.)

+ clone the core repo

+ cd to it

run: <path>/tinbuild2 -p gerrit -g refs/changes/<n>/<m>/<p>

the value of the -g argument can be cut and pasted from gerrit itself.
in the detail view of a given 'commit' under the 'Download' section of
a given patchset.

This will:
fetch the repot at the level of the said patchset,
otify gerrit (in the review message) that you tinderbox is starting a
build on this patchset (for the benefit of other, so they know that
this is pending)
build clean + bluid
set Verify +1 / -1 on gerrit for that patchset depending on the succes
or failure
restore your core repo to the branch it was before we started (master
in this case if you followed the instruction above)


(*) Note when doing a gerrit buildtest, tinbuild2 will not send
email.. so you do not need to have a working SMTP configuration in
~/.tinbuild/config/gerrit.cfg

Norbert