Rename "master" branch to "main" for glibc 2.35 release.

[Carlos O'Donell <carlos@redhat.com>]

In June 2020 I started this thread for glibc 2.32:
https://sourceware.org/pipermail/libc-alpha/2020-June/115579.html
to discuss a branch name change.

Thanks to Paul, Florian, Joseph and DJ who provided feedback on
the initial discussion.

As of today the default Gitlab new branch is "main" (Gitlab 14 or later).

As of today the default Github new branch is "main".

With Git 2.28+ the default branch name is configurable.

Git has added this advisory text:
~~~
 562 static const char default_branch_name_advice[] = N_(
 563 "Using '%s' as the name for the initial branch. This default branch name\n"
 564 "is subject to change. To configure the initial branch name to use in all\n"
 565 "of your new repositories, which will suppress this warning, call:\n"
 566 "\n"
 567 "\tgit config --global init.defaultBranch <name>\n"
 568 "\n"
 569 "Names commonly chosen instead of 'master' are 'main', 'trunk' and\n"
 570 "'development'. The just-created branch can be renamed via this command:\n"
 571 "\n"
 572 "\tgit branch -m <name>\n"
~~~

My proposal is to rename the development and release branches at the point
that glibc 2.35 branches:

* master -> main

* release/2.35/master -> release/2.35/main

No alias would be provided for the master branch; we would immediately
start using 'main' as the development branch.

I am looking for consensus on this proposal.

Details:
========

Given the feedback from the last proposal I have done the required
review to ensure the transition can happen smoothly within our own
infrastructure.

This proposal is only about the development branch and upcoming release
branch. All historical branches remain at their existing names.

I have reviewed the git hooks, and we would need to make changes to the
following files after the transition:
* hooks-bin/email-to-bugzilla-filtered
* hooks-bin/post-receive

Our hooks implementation (AdaCore's hooks) is neutral on the branch name
used. There are two instances where 'master' is removed for '' to
shorten an email message because it's the "default", but that should not
be a blocker for our transition.

There are changes required to our project.config in refs/meta/config:
* reject-merge-commits for 'main' needs to be set.
* allow-non-fast-forward must be adjusted for 'main'.

Wiki pages that need changing immediately:
* https://sourceware.org/glibc/wiki/GlibcGit

How to update a renamed upstream branch:
# Rename old branch to new branch.
$ git branch -m OLD-BRANCH-NAME NEW-BRANCH-NAME
# Fetch upstream origin to get new branch name.
$ git fetch origin
# Set upstream to thew new renamed branch.
$ git branch -u origin/NEW-BRANCH-NAME NEW-BRANCH-NAME
# Set the default branch for origin automatically
$ git remote set-head origin -a
# Prune the old references away.
$ git remote prune origin

Website needs updating immediately:
* https://www.gnu.org/software/libc/sources.html
* https://www.gnu.org/software/libc/started.html

All other wiki, web, and source references can be adjusted over time.

-- 
Cheers,
Carlos.