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>

builtin/checkout.c

@@ -916,6 +916,17 @@ static int parse_branchname_arg(int argc, const char **argv,
 	return argcount;
 }
 
+static int switch_unborn_to_new_branch(struct checkout_opts *opts)
+{
+	int status;
+	struct strbuf branch_ref = STRBUF_INIT;
+
+	strbuf_addf(&branch_ref, "refs/heads/%s", opts->new_branch);
+	status = create_symref("HEAD", branch_ref.buf, "checkout -b");
+	strbuf_release(&branch_ref);
+	return status;
+}
+
 int cmd_checkout(int argc, const char **argv, const char *prefix)
 {
 	struct checkout_opts opts;
@@ -1089,5 +1100,13 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 	if (opts.writeout_stage)
 		die(_("--ours/--theirs is incompatible with switching branches."));
 
+	if (!new.commit) {
+		unsigned char rev[20];
+		int flag;
+
+		resolve_ref("HEAD", rev, 0, &flag);
+		if ((flag & REF_ISSYMREF) && is_null_sha1(rev))
+			return switch_unborn_to_new_branch(&opts);
+	}
 	return switch_branches(&opts, &new);
 }

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