repository: do not initialize HEAD if it's provided by templates

When using templates to initialize a git repository, then git-init(1)
will copy over all contents of the template directory. These will be
preferred over the default ones created by git-init(1). While we mostly
do the same, there is the exception of "HEAD". While we do copy over the
template's HEAD file, afterwards we'll immediately re-initialize its
contents with either the default "ref: refs/origin/master" or the init
option's `initial_head` field.

Let's fix the inconsistency with upstream git-init(1) by not overwriting
the template HEAD, but only if the user hasn't set `opts.initial_head`.
If the `initial_head` field has been supplied, we should use that
indifferent from whether the template contained a HEAD file or not. Add
tests to verify we correctly use the template directory's HEAD file and
that `initial_head` overrides the template.

src/repository.c

@@ -2056,7 +2056,8 @@ int git_repository_init_ext(
 	const char *given_repo,
 	git_repository_init_options *opts)
 {
-	git_buf repo_path = GIT_BUF_INIT, wd_path = GIT_BUF_INIT, common_path = GIT_BUF_INIT;
+	git_buf repo_path = GIT_BUF_INIT, wd_path = GIT_BUF_INIT,
+		common_path = GIT_BUF_INIT, head_path = GIT_BUF_INIT;
 	const char *wd;
 	int error;
 
@@ -2086,6 +2087,15 @@ int git_repository_init_ext(
 	} else {
 		if ((error = repo_init_structure(repo_path.ptr, wd, opts)) < 0 ||
 		    (error = repo_init_config(repo_path.ptr, wd, opts->flags, opts->mode)) < 0 ||
+		    (error = git_buf_joinpath(&head_path, repo_path.ptr, GIT_HEAD_FILE)) < 0)
+			goto out;
+
+		/*
+		 * Only set the new HEAD if the file does not exist already via
+		 * a template or if the caller has explicitly supplied an
+		 * initial HEAD value.
+		 */
+		if ((!git_path_exists(head_path.ptr) || opts->initial_head) &&
 		    (error = git_repository_create_head(repo_path.ptr, opts->initial_head)) < 0)
 			goto out;
 	}
@@ -2098,6 +2108,7 @@ int git_repository_init_ext(
 		goto out;
 
 out:
+	git_buf_dispose(&head_path);
 	git_buf_dispose(&common_path);
 	git_buf_dispose(&repo_path);
 	git_buf_dispose(&wd_path);

tests/repo/template.c

@@ -249,6 +249,41 @@ void test_repo_template__extended_with_template_and_shared_mode(void)
 	validate_templates(_repo, "template");
 }
 
+void test_repo_template__templated_head_is_used(void)
+{
+	git_repository_init_options opts = GIT_REPOSITORY_INIT_OPTIONS_INIT;
+	git_buf head = GIT_BUF_INIT;
+
+	opts.flags = GIT_REPOSITORY_INIT_MKPATH | GIT_REPOSITORY_INIT_EXTERNAL_TEMPLATE;
+
+	setup_templates("template", true);
+	cl_git_mkfile("template/HEAD", "foobar\n");
+	setup_repo("repo", &opts);
+
+	cl_git_pass(git_futils_readbuffer(&head, "repo/.git/HEAD"));
+	cl_assert_equal_s("foobar\n", head.ptr);
+
+	git_buf_dispose(&head);
+}
+
+void test_repo_template__initial_head_option_overrides_template_head(void)
+{
+	git_repository_init_options opts = GIT_REPOSITORY_INIT_OPTIONS_INIT;
+	git_buf head = GIT_BUF_INIT;
+
+	opts.flags = GIT_REPOSITORY_INIT_MKPATH | GIT_REPOSITORY_INIT_EXTERNAL_TEMPLATE;
+	opts.initial_head = "manual";
+
+	setup_templates("template", true);
+	cl_git_mkfile("template/HEAD", "foobar\n");
+	setup_repo("repo", &opts);
+
+	cl_git_pass(git_futils_readbuffer(&head, "repo/.git/HEAD"));
+	cl_assert_equal_s("ref: refs/heads/manual\n", head.ptr);
+
+	git_buf_dispose(&head);
+}
+
 void test_repo_template__empty_template_path(void)
 {
 	git_repository_init_options opts = GIT_REPOSITORY_INIT_OPTIONS_INIT;