init: provide useful advice about init.defaultBranch

To give ample warning for users wishing to override Git's the fall-back
for an unconfigured `init.defaultBranch` (in case we decide to change it
in a future Git version), let's introduce some advice that is shown upon
`git init` when that value is not set.

Note: two test cases in Git's test suite want to verify that the
`stderr` output of `git init` is empty. It is now necessary to suppress
the advice, we now do that via the `init.defaultBranch` setting. While
not strictly necessary, we also set this to `false` in
`test_create_repo()`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -562,6 +562,19 @@ void expand_ref_prefix(struct strvec *prefixes, const char *prefix)
 		strvec_pushf(prefixes, *p, len, prefix);
 }
 
+static const char default_branch_name_advice[] = N_(
+"Using '%s' as the name for the initial branch. This default branch name\n"
+"is subject to change. To configure the initial branch name to use in all\n"
+"of your new repositories, which will suppress this warning, call:\n"
+"\n"
+"\tgit config --global init.defaultBranch <name>\n"
+"\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"
+);
+
 char *repo_default_branch_name(struct repository *r, int quiet)
 {
 	const char *config_key = "init.defaultbranch";
@@ -574,8 +587,11 @@ char *repo_default_branch_name(struct repository *r, int quiet)
 	else if (repo_config_get_string(r, config_key, &ret) < 0)
 		die(_("could not retrieve `%s`"), config_display_key);
 
-	if (!ret)
+	if (!ret) {
 		ret = xstrdup("master");
+		if (!quiet)
+			advise(_(default_branch_name_advice), ret);
+	}
 
 	full_ref = xstrfmt("refs/heads/%s", ret);
 	if (check_refname_format(full_ref, 0))

t/t0001-init.sh

@@ -163,7 +163,7 @@ test_expect_success 'reinit' '
 	(
 		mkdir again &&
 		cd again &&
-		git init >out1 2>err1 &&
+		git -c init.defaultBranch=initial init >out1 2>err1 &&
 		git init >out2 2>err2
 	) &&
 	test_i18ngrep "Initialized empty" again/out1 &&
@@ -558,6 +558,13 @@ test_expect_success 'overridden default initial branch name (config)' '
 	grep nmb actual
 '
 
+test_expect_success 'advice on unconfigured init.defaultBranch' '
+	GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME= git -c color.advice=always \
+		init unconfigured-default-branch-name 2>err &&
+	test_decode_color <err >decoded &&
+	test_i18ngrep "<YELLOW>hint: " decoded
+'
+
 test_expect_success 'overridden default main branch name (env)' '
 	test_config_global init.defaultBranch nmb &&
 	GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=env git init main-branch-env &&

t/t1510-repo-setup.sh

@@ -79,7 +79,7 @@ setup_repo () {
 	name=$1 worktreecfg=$2 gitfile=$3 barecfg=$4 &&
 	sane_unset GIT_DIR GIT_WORK_TREE &&
 
-	git init "$name" &&
+	git -c init.defaultBranch=initial init "$name" &&
 	maybe_config "$name/.git/config" core.worktree "$worktreecfg" &&
 	maybe_config "$name/.git/config" core.bare "$barecfg" &&
 	mkdir -p "$name/sub/sub" &&

t/test-lib-functions.sh

@@ -1202,7 +1202,9 @@ test_create_repo () {
 	mkdir -p "$repo"
 	(
 		cd "$repo" || error "Cannot setup test environment"
-		"${GIT_TEST_INSTALLED:-$GIT_EXEC_PATH}/git$X" init \
+		"${GIT_TEST_INSTALLED:-$GIT_EXEC_PATH}/git$X" -c \
+			init.defaultBranch="${GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME-master}" \
+			init \
 			"--template=$GIT_BUILD_DIR/templates/blt/" >&3 2>&4 ||
 		error "cannot run git init -- have you built things yet?"
 		mv .git/hooks .git/hooks-disabled