Default branch renamed (with minor edit to history)

The work outlined in Minor re-writing of history and moving from master to main for default branch - #11 by tacaswell has been completed. The main actions are:

  1. the default branch for Matplotlib development has been renamed from mastermain
  2. 3 commits have been removed from the default history
  3. The old master branch is temporarily available as old_master and will be removed once the dust has fully settled

Any PR that includes the 3 removed commits will need to be rebased onto the new default branch. There are several ways to accomplish this, but we recommend (assuming that you call the matplotlib/matplotlib remote "upstream"

git remote update
git checkout main
git merge --ff-only upstream/main
git checkout YOUR_BRANCH
git rebase --onto=main upstream/old_master
# git rebase -i main # if you prefer
git push --force-with-lease   # assuming you are tracking your branch

If you follow the gitwash instructions you may need to replace main with placeholder as

git rebase --onto=placeholder upstream/old_master
# git rebase -i placeholder  # if you prefer

and if you do not have a tracking branch configured you may need to do, assuming the remote for you fork is called origin

git push --force-with-lease origin YOUR_BRANCH

Following the directions from the github UI may result in these commits coming back, in that case the fix is

git checkout main   
# git checkout placeholder   # if following gitwash
git reset --hard upstream/main

Ensure that when you do this you have no commits on the branch or changes in the working tree that you want to keep as they will be eliminated.

The log of the force push is:

$ git push DANGER --force-with-lease main:main
Enumerating objects: 5941, done.
Counting objects: 100% (3462/3462), done.
Delta compression using up to 8 threads
Compressing objects: 100% (1300/1300), done.
Writing objects: 100% (2876/2876), 766.75 KiB | 21.30 MiB/s, done.
Total 2876 (delta 1983), reused 1962 (delta 1564), pack-reused 0
remote: Resolving deltas: 100% (1983/1983), completed with 296 local objects.
 + f6e0ee49c5...a1eef38f6f main -> main (forced update)