Add helpful advice about init.defaultBranch

It is our designated intention
[https://sfconservancy.org/news/2020/jun/23/gitbranchname/] to "to explore changing the name of the first branch created automatically for new repositories away from ‘master’". Further, we promised that we "will minimize disruption for Git's users and will include appropriate deprecation periods".

To that end, we first clarify git init's documentation (a lightly edited version of Junio's patch
[https://lore.kernel.org/git/[email protected]]), and then introduce a message giving advice to users running git init that the fall-back for init.defaultBranch is subject to change, and how to override Git's fall-back manually.

The beauty of this patch series, if I may say so, is that it does not commit us to any concrete plan, but opens the door for all options regarding the default branch name.

Changes since v4:

* Reworded the documentation to weaken the "will be changed in the future" to "is subject to change".
* Reworded the advice to explicitly mention "instead of 'master'".

Changes since v3:

* Kicked out advice.defaultBranchName again. The advice can now only be suppressed by setting init.defaultBranch.

Changes since v2:

* git branch -m <initial> <renamed> now works, too
* The config setting advice.defaultBranchName was introduced to allow suppressing this warning specifically
* The advice now first talks about the current default branch name, then how to suppress the advice, and only then explains how to override the default branch name (and that that will also prevent the message from being shown)

Changes since v1:

* Grammar fix
* Reworded advice and commit messages
* test_create_repo already respected GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME, but now does so more obviously
* Allowed git branch -m <name> to rename unborn branches, so that the advice actually works

Johannes Schindelin (4):
init: document init.defaultBranch better branch -m: allow renaming a yet-unborn branch get_default_branch_name(): prepare for showing some advice
init: provide useful advice about init.defaultBranch

 Documentation/git-init.txt | 11 +++++++----
 builtin/branch.c           |  4 +++-
 builtin/clone.c            |  2 +-
 builtin/init-db.c          |  8 +++++---
 refs.c                     | 24 ++++++++++++++++++++----
 refs.h                     |  4 ++--
 remote.c                   |  5 +++--
 t/t0001-init.sh            | 17 ++++++++++++++++-
 t/t1510-repo-setup.sh      |  2 +-
 t/test-lib-functions.sh    |  4 +++-
 10 files changed, 61 insertions(+), 20 deletions(-)

base-commit: faefdd61ec7c7f6f3c8c9907891465ac9a2a1475
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-921%2Fdscho%2Finit.defaultBranch-advice-v5
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-921/dscho/init.defaultBranch-advice-v5
Pull-Request: https://github.com/git/git/pull/921

Range-diff vs v4:

1: 483e4903491 ! 1: ce35cf3da65 init: document init.defaultBranch better
@@ Documentation/git-init.txt: If this is reinitialization, the repository will be
-If not specified, fall back to the default name: master.
+Use the specified name for the initial branch in the newly created +repository. If not specified, fall back to the default name (currently
-+master, but that will be changed in the future; the name can be customized -+via the init.defaultBranch configuration variable).
++master, but this is subject to change in the future; the name can be ++customized via the init.defaultBranch configuration variable).

--shared[=(false|true|umask|group|all|world|everybody|0xxx)]::

 2:  6cbd6692c6a = 2:  ac7b352a0ba branch -m: allow renaming a yet-unborn branch
 3:  5fc15f7b9ad = 3:  aba72340c87 get_default_branch_name(): prepare for showing some advice
 4:  2f21c505d56 ! 4:  ce26446b1a2 init: provide useful advice about init.defaultBranch
     @@ refs.c: void expand_ref_prefix(struct strvec *prefixes, const char *prefix)
      +"\n"
      +"\tgit config --global init.defaultBranch <name>\n"
      +"\n"
     -+"Common names are 'main', 'trunk' and 'development'. The initial branch\n"
     -+"that was created can be renamed via this command:\n"
     ++"Names commonly chosen instead of 'master' are 'main', 'trunk' and\n"
     ++"'development'. The just-created branch can be renamed via this command:\n"
      +"\n"
      +"\tgit branch -m <name>\n"
      +);

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