clone: use configured default branch name when appropriate

When cloning a repository without any branches, Git chooses a default
branch name for the as-yet unborn branch.

As part of the implicit initialization of the local repository, Git just
learned to respect `init.defaultBranch` to choose a different initial
branch name. We now really want that branch name to be used as a
fall-back.

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

Documentation/config/init.txt

@@ -3,5 +3,5 @@ init.templateDir::
 	(See the "TEMPLATE DIRECTORY" section of linkgit:git-init[1].)
 
 init.defaultBranch::
-	Allows overriding the default branch name when initializing
-	a new repository.
+	Allows overriding the default branch name e.g. when initializing
+	a new repository or when cloning an empty repository.

builtin/clone.c

@@ -1264,9 +1264,13 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 		remote_head_points_at = NULL;
 		remote_head = NULL;
 		option_no_checkout = 1;
-		if (!option_bare)
-			install_branch_config(0, "master", option_origin,
-					      "refs/heads/master");
+		if (!option_bare) {
+			const char *branch = git_default_branch_name();
+			char *ref = xstrfmt("refs/heads/%s", branch);
+
+			install_branch_config(0, branch, option_origin, ref);
+			free(ref);
+		}
 	}
 
 	write_refspec_config(src_ref_prefix, our_head_points_at,

t/t5606-clone-options.sh

@@ -35,4 +35,19 @@ test_expect_success 'redirected clone -v does show progress' '
 
 '
 
+test_expect_success 'chooses correct default initial branch name' '
+	git init --bare empty &&
+	git -c init.defaultBranch=up clone empty whats-up &&
+	test refs/heads/up = $(git -C whats-up symbolic-ref HEAD) &&
+	test refs/heads/up = $(git -C whats-up config branch.up.merge)
+'
+
+test_expect_success 'guesses initial branch name correctly' '
+	git init --initial-branch=guess initial-branch &&
+	test_commit -C initial-branch no-spoilers &&
+	git -C initial-branch branch abc guess &&
+	git clone initial-branch is-it &&
+	test refs/heads/guess = $(git -C is-it symbolic-ref HEAD)
+'
+
 test_done