Merge branch 'db/maint-checkout-b' into maint

* db/maint-checkout-b:
  Check early that a new branch is new and valid

builtin-checkout.c

@@ -565,6 +565,18 @@ no_reference:
 		return checkout_paths(source_tree, pathspec);
 	}
 
+	if (opts.new_branch) {
+		struct strbuf buf;
+		strbuf_init(&buf, 0);
+		strbuf_addstr(&buf, "refs/heads/");
+		strbuf_addstr(&buf, opts.new_branch);
+		if (!get_sha1(buf.buf, rev))
+			die("git checkout: branch %s already exists", opts.new_branch);
+		if (check_ref_format(buf.buf))
+			die("git checkout: we do not like '%s' as a branch name.", opts.new_branch);
+		strbuf_release(&buf);
+	}
+
 	if (new.name && !new.commit) {
 		die("Cannot switch branch to a non-commit.");
 	}

t/t7201-co.sh

@@ -359,4 +359,14 @@ test_expect_success 'checkout an unmerged path should fail' '
 	test_cmp sample file
 '
 
+test_expect_success 'failing checkout -b should not break working tree' '
+	git reset --hard master &&
+	git symbolic-ref HEAD refs/heads/master &&
+	test_must_fail git checkout -b renamer side^ &&
+	test $(git symbolic-ref HEAD) = refs/heads/master &&
+	git diff --exit-code &&
+	git diff --cached --exit-code
+
+'
+
 test_done