breaking-changes: switch default branch to main

Since 1296cbe4b46 (init: document `init.defaultBranch` better,
2020-12-11) "git-init.adoc" has advertised that the default name
of the initial branch may change in the future. The name "main"
is chosen to match the default used by the big Git forge web sites.

The advice printed when init.defaultBranch is not set is updated
to say that the default will change to "main" in Git 3.0. Building
with WITH_BREAKING_CHANGES enabled removes the advice and changes
the default branch name to "main". The code in guess_remote_head()
that looks for "refs/heads/master" is left unchanged as that is only
called when the remote server does not support the symref capability
in the v0 protocol or the symref extension to the ls-refs list in the
v2 protocol. Such an old server is more likely to be using "master"
as the default branch name.

With the exception of the "git-init.adoc" the documentation is left
unchanged. I had hoped to parameterize the name of the default branch
by using an asciidoc attribute. Unfortunately attribute expansion
is inhibited by backticks and we use backticks to mark up ref names
so that idea does not work. As the changes to git-init.adoc show
inserting ifdef's around each instance of the branch name "master"
is cumbersome and makes the documentation sources harder to read.

Apart from "git-init.adoc" there are some other files where "master" is
used as the name of the initial branch rather than as an example of a
branch name such as "user-manual.adoc" and "gitcore-tutorial.adoc". The
name appears a lot in those so updating it with ifdef's is not really
practical. We can update that document in the 3.0 release cycle. The
other documentation where master is used as an example branch name
can be gradually converted over time.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 15 insertions, 3 deletions

diff --git a/Documentation/BreakingChanges.adoc b/Documentation/BreakingChanges.adoc
index f8d2eba061..f4e11c8865 100644
--- a/Documentation/BreakingChanges.adoc
+++ b/Documentation/BreakingChanges.adoc
@@ -165,6 +165,12 @@ A prerequisite for this change is that the ecosystem is ready to support the
 "reftable" format. Most importantly, alternative implementations of Git like
 JGit, libgit2 and Gitoxide need to support it.
 
+* In new repositories, the default branch name will be `main`. We have been
+  warning that the default name will change since 675704c74dd (init:
+  provide useful advice about init.defaultBranch, 2020-12-11).  The new name
+  matches the default branch name used in new repositories by many of the
+  big Git forges.
+
 === Removals
 
 * Support for grafting commits has long been superseded by git-replace(1).
diff --git a/Documentation/git-init.adoc b/Documentation/git-init.adoc
index a0dffba665..bab99b9b47 100644
--- a/Documentation/git-init.adoc
+++ b/Documentation/git-init.adoc
@@ -77,9 +77,15 @@ If this is a reinitialization, the repository will be moved to the specified pat
 `-b <branch-name>`::
 `--initial-branch=<branch-name>`::
 Use _<branch-name>_ for the initial branch in the newly created
-repository.  If not specified, fall back to the default name (currently
-`master`, but this is subject to change in the future; the name can be
-customized via the `init.defaultBranch` configuration variable).
+repository.  If not specified, fall back to the default name
+ifndef::with-breaking-changes[]
+(currently `master`, but this will change to `main` when Git 3.0 is released).
+endif::with-breaking-changes[]
+ifdef::with-breaking-changes[]
+`main`.
+endif::with-breaking-changes[]
+The default name can be customized via the `init.defaultBranch` configuration
+variable.
 
 `--shared[=(false|true|umask|group|all|world|everybody|<perm>)]`::