git checkout -b: allow switching out of an unborn branch

Running "git checkout -b another" immediately after "git init" when you do
not even have a commit on 'master' fails with:

    $ git checkout -b another
    fatal: You are on a branch yet to be born

This is unnecessary, if we redefine "git checkout -b $name" that does not
take any $start_point (which has to be a commit) as "I want to check out a
new branch $name from the state I am in".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2015-checkout-unborn.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-test_description='checkout from unborn branch protects contents'
+test_description='checkout from unborn branch'
 . ./test-lib.sh
 
 test_expect_success 'setup' '
@@ -37,4 +37,13 @@ test_expect_success 'checkout from unborn merges identical index contents' '
 	git checkout -b new origin
 '
 
+test_expect_success 'checking out another branch from unborn state' '
+	git checkout --orphan newroot &&
+	git checkout -b anothername &&
+	test_must_fail git show-ref --verify refs/heads/newroot &&
+	git symbolic-ref HEAD >actual &&
+	echo refs/heads/anothername >expect &&
+	test_cmp expect actual
+'
+
 test_done