Git: Fix initial clone with gerrit changes

git clone --branch only works with refs under refs/heads/* and
refs/tags/*, not arbitrary refs. But the Gerrit step will give us a
ref of the form refs/changes/*, since that is where gerrit stores the
submitted changes for us to build.

There appears to be no way to get an arbitrary ref with 'git clone',
as you can with --branch; at least, none I can see. So instead, just
do a regular clone, and run the normal _fetch routine afterwards,
which will fetch the relevant ref and check it out for us.

This bug is mentioned in bug #2485, though #2485 itself is a different
bug.
GerritStatusPush: Fix labels with gerrit >=2.6

Fixes #2550. Gerrit versions 2.6 and newer do not accept the
--verified option if the Verified label does not exist in All-Projects
in the gerrit configuration. Instead, we must give the --label option.
So, use --label for --verified and --code-review so we can work with
all gerrit installations.

While the --code-review option does still work, so does the --label
approach. It seems more likely that --label is more likely to work in
the future than --code-review, so we'll just go with the safer route.

In order to still support older gerrit installations (which do not
support the --label option), query the gerrit installation version
whenever we submit a review. Cache this version query for 10 minutes,
so we do not ask the gerrit server every single time for its version.
GerritStatusPush: Add per-buildset status updates

Currently GerritStatusPush will submit an update/review to gerrit
immediately when each builder in a buildset finishes building. It is
usually more useful to just get one update, containing information
about all of the builds in that buildset. So, allow GerritStatusPush
to do that, configurable via the new summaryCB constructor argument. A
default summaryCB is also provided: defaultSummaryCB.

If neither reviewCB nor summaryCB are explicitly provided in the
configuration, default to sending out the new "summary" reviews.
However, if reviewCB was explicitly provided, default to sending out
the old-style per-build reviews, and not sending out the new "summary"
reviews. This allows for older configurations to not see an unexpected
change in behavior, but completely new configurations are likely to
see the probably more useful "summary" reviews.

Since much of this functionality is already implemented in
MailNotifier, extract the relevant code into the new
BuildSetSummaryNotifierMixin, and make both MailNotifier and
GerritStatusPush use it. Document the new mixin class.

Also add tests for GerritStatusPush, to test the new behavior, and for
BuildSetSummaryNotifierMixin, to test that the mixin itself works
properly. Modify the unit tests for MailNotifier to accomodate the
refactoring.
Merge adeason/buildbot:gerrit-fixes (PR #899)

Conflicts:
	master/docs/relnotes/index.rst

+autopep8