repository: restrict checking out checked out branches

If a branch is already checked out in a working tree we are not
allowed to check out that branch in another repository. Introduce
this restriction when setting a repository's HEAD.

src/repository.c

@@ -2529,6 +2529,12 @@ int git_repository_set_head(
 	if (error < 0 && error != GIT_ENOTFOUND)
 		goto cleanup;
 
+	if (ref && current->type == GIT_REF_SYMBOLIC && git__strcmp(current->target.symbolic, ref->name) &&
+	    git_reference_is_branch(ref) && git_branch_is_checked_out(ref)) {
+		error = -1;
+		goto cleanup;
+	}
+
 	if (!error) {
 		if (git_reference_is_branch(ref)) {
 			error = git_reference_symbolic_create(&new_head, repo, GIT_HEAD_FILE,

tests/worktree/refs.c

@@ -68,6 +68,41 @@ void test_worktree_refs__read_head(void)
 	git_reference_free(head);
 }
 
+void test_worktree_refs__set_head_fails_when_worktree_wants_linked_repos_HEAD(void)
+{
+	git_reference *head;
+
+	cl_git_pass(git_repository_head(&head, fixture.repo));
+	cl_git_fail(git_repository_set_head(fixture.worktree, git_reference_name(head)));
+
+	git_reference_free(head);
+}
+
+void test_worktree_refs__set_head_fails_when_main_repo_wants_worktree_head(void)
+{
+	git_reference *head;
+
+	cl_git_pass(git_repository_head(&head, fixture.worktree));
+	cl_git_fail(git_repository_set_head(fixture.repo, git_reference_name(head)));
+
+	git_reference_free(head);
+}
+
+void test_worktree_refs__set_head_works_for_current_HEAD(void)
+{
+	git_reference *head;
+
+	cl_git_pass(git_repository_head(&head, fixture.repo));
+	cl_git_pass(git_repository_set_head(fixture.repo, git_reference_name(head)));
+
+	git_reference_free(head);
+}
+
+void test_worktree_refs__set_head_fails_when_already_checked_out(void)
+{
+	cl_git_fail(git_repository_set_head(fixture.repo, "refs/heads/testrepo-worktree"));
+}
+
 void test_worktree_refs__delete_fails_for_checked_out_branch(void)
 {
        git_reference *branch;