init: allow setting the default for the initial branch name via the config

We just introduced the command-line option
`--initial-branch=<branch-name>` to allow initializing a new repository
with a different initial branch than the hard-coded one.

To allow users to override the initial branch name more permanently
(i.e. without having to specify the name manually for each and every
`git init` invocation), let's introduce the `init.defaultBranch` config
setting.
Helped-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Helped-by: NDerrick Stolee <dstolee@microsoft.com>
Signed-off-by: NDon Goodman-Wilson <don@goodman-wilson.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/config/init.txt

 init.templateDir::
 	Specify the directory from which templates will be copied.
 	(See the "TEMPLATE DIRECTORY" section of linkgit:git-init[1].)
+
+init.defaultBranch::
+	Allows overriding the default branch name when initializing
+	a new repository.

builtin/init-db.c

@@ -269,7 +269,7 @@ static int create_default_files(const char *template_path,
 		char *ref;
 
 		if (!initial_branch)
-			initial_branch = "master";
+			initial_branch = git_default_branch_name();
 
 		ref = xstrfmt("refs/heads/%s", initial_branch);
 		if (check_refname_format(ref, 0) < 0)

refs.c

@@ -560,6 +560,36 @@ void expand_ref_prefix(struct argv_array *prefixes, const char *prefix)
 		argv_array_pushf(prefixes, *p, len, prefix);
 }
 
+char *repo_default_branch_name(struct repository *r)
+{
+	const char *config_key = "init.defaultbranch";
+	const char *config_display_key = "init.defaultBranch";
+	char *ret = NULL, *full_ref;
+
+	if (repo_config_get_string(r, config_key, &ret) < 0)
+		die(_("could not retrieve `%s`"), config_display_key);
+
+	if (!ret)
+		ret = xstrdup("master");
+
+	full_ref = xstrfmt("refs/heads/%s", ret);
+	if (check_refname_format(full_ref, 0))
+		die(_("invalid branch name: %s = %s"), config_display_key, ret);
+	free(full_ref);
+
+	return ret;
+}
+
+const char *git_default_branch_name(void)
+{
+	static char *ret;
+
+	if (!ret)
+		ret = repo_default_branch_name(the_repository);
+
+	return ret;
+}
+
 /*
  * *string and *len will only be substituted, and *string returned (for
  * later free()ing) if the string passed in is a magic short-hand form

refs.h

@@ -154,6 +154,15 @@ int repo_dwim_log(struct repository *r, const char *str, int len, struct object_
 int dwim_ref(const char *str, int len, struct object_id *oid, char **ref);
 int dwim_log(const char *str, int len, struct object_id *oid, char **ref);
 
+/*
+ * Retrieves the default branch name for newly-initialized repositories.
+ *
+ * The return value of `repo_default_branch_name()` is an allocated string. The
+ * return value of `git_default_branch_name()` is a singleton.
+ */
+const char *git_default_branch_name(void);
+char *repo_default_branch_name(struct repository *r);
+
 /*
  * A ref_transaction represents a collection of reference updates that
  * should succeed or fail together.

t/t0001-init.sh

@@ -477,4 +477,17 @@ test_expect_success '--initial-branch' '
 	grep hello actual
 '
 
+test_expect_success 'overridden default initial branch name (config)' '
+	test_config_global init.defaultBranch nmb &&
+	git init initial-branch-config &&
+	git -C initial-branch-config symbolic-ref HEAD >actual &&
+	grep nmb actual
+'
+
+test_expect_success 'invalid default branch name' '
+	test_config_global init.defaultBranch "with space" &&
+	test_must_fail git init initial-branch-invalid 2>err &&
+	test_i18ngrep "invalid branch name" err
+'
+
 test_done