Allow Rally to handle master -> main migration of Elasticsearch repo

Elasticsearch is moving to main (https://github.com/elastic/elasticsearch/issues/76950) and even has plans to remove the master branch. How can we handle this transition in Rally?

Most of it is documentation/comments:

• README.rst “always adapts accordingly to the latest master version”
• docs/developing.rst: change “build setup requirements” URL to master (even if GitHub will redirect)
• docs/car.rst: The master branch of team repos needs to work with the main branch of Elasticsearch
• docs/track.rst: The master branch needs to work with the latest main branch of Elasticsearch (and various mentions of master below - ⚠️ this file also mentions the master branch of the track repos, this should stay)
• docs/elasticsearch_plugin.rst: By default Rally assumes that you want to benchmark the latest main version of Elasticsearch.
• docs/command_line_reference.rst: --revision=latest: use the HEAD revision of from origin/main
• esrally/racecontrol:BenchmarkCoordinator.setup: change master to main (or latest?)
• esrally/rally.py: in install_parser change master to main (or latest?)
• esrally/track/loader: in tracks docstring change master to main (or latest?)

But there are changes to code too.

• esrally/mechanic/supplier: SourceRepository should call git.pull and git.pull_ts with branch="main" for Elasticsearch but not plugins
• if we have an existing clean checkout, we convert it to main

Teams/tracks don’t need to change, we’ll move to main eventually but it’s out of scope here.

Open questions:

• Should we assume that all plugins use the main branch too? The only one I’m aware of that has not moved: https://github.com/BigDataBoutique/elasticsearch-repository-swift. If no, then SourceRepository needs to know the default branch. Should it be configured? I’m not sure how much effort we should put into plugins given the official ones got integrated in Elasticsearch itself.
• How do we handle existing checkouts? “if the branch is master and clean, then git fetch origin and git checkout main”?

[pquentin]

Yes, all the other repos are already migrated, they only needed doc changes.

However, if this breaks our nightly benchmarks we’ll have to revert. They run at 6PM UTC, so let’s wait until tomorrow to be sure.

[pquentin]

In the past it looks like we had many official plugins in their own repos, but not so much now. The only one I was able to find is https://github.com/BigDataBoutique/elasticsearch-repository-swift. We’ll keep using master for plugins by default for now, thanks.