Compare breeze verify with main on the apache/airflow remote by default

The default --base-ref was the local main branch. In a fork it is often
behind the main the branch has merged (a GitHub "Update branch" or a
conflict fix), and then every merged-in commit counted as a change:
3287 files instead of the 21 the PR changes.

The default is now main on the remote whose URL points at apache/airflow,
preferring upstream, which is the base GitHub compares the PR with.
Without such a remote it falls back to the local main and says so.

The local copy of that remote can be stale too, after "Update branch"
without a fetch. breeze verify now warns when a merge on the branch
brought in commits the base does not have. With --json the warnings go
to stderr so stdout stays parseable.

Author: Andrushika

dev/breeze/doc/03_developer_tasks.rst

@@ -324,12 +324,19 @@ Finding out what CI will run for your change
 and lists the commands to run locally for the change. Nothing is executed. The only non-zero exit
 is a usage error such as a base ref git cannot resolve.
 
+By default ``--base-ref`` is ``main`` on the git remote that points at apache/airflow (``upstream``
+if several do), the same base GitHub compares a PR with. A local ``main`` that is behind the
+``main`` your branch has merged would count every merged-in commit as your change. Without such a
+remote, ``breeze verify`` falls back to the local ``main`` branch and says so. It also warns when
+the branch has merged commits the base does not have, for example after GitHub's "Update branch"
+when the remote was not fetched since. With ``--json`` the warnings go to stderr.
+
 .. code-block:: bash
 
      breeze verify
      breeze verify --json
      breeze verify --full
-     breeze verify --base-ref upstream/main
+     breeze verify --base-ref main
 
 Each row says what kind of check it is, whether it runs on the host or needs Docker and the
 CI image (``breeze``), and the exact command. Jobs CI runs on every PR regardless of the change

dev/breeze/doc/images/output_verify.svg

@@ -1 +1 @@
-643ed2129027a31d158a80470b531b79
+af28dee252e39588e825b2fef52146f0