checkout: do not corrupt HEAD on empty repo

In abe19980 ("git checkout -b: allow switching out of an unborn
branch"), a code-path overly-optimisticly assumed that a
branch-name was specified. This is not always the case, and as
a result a NULL-pointer was attempted printed to .git/HEAD.

This could lead to at least two different failure modes:
 1) vsnprintf formated the NULL-string as something useful (e.g
    "(null)")
 2) vsnprintf crashed

Neither were very convenient for formatting a new HEAD-reference.

To fix this, reintroduce some strictness so we only take this
new codepath if a banch-name was specified.
Signed-off-by: NErik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -1100,7 +1100,7 @@ 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) {
+	if (!new.commit && opts.new_branch) {
 		unsigned char rev[20];
 		int flag;
 

t/t2015-checkout-unborn.sh

@@ -46,4 +46,15 @@ test_expect_success 'checking out another branch from unborn state' '
 	test_cmp expect actual
 '
 
+test_expect_success 'checking out in a newly created repo' '
+	test_create_repo empty &&
+	(
+		cd empty &&
+		git symbolic-ref HEAD >expect &&
+		test_must_fail git checkout &&
+		git symbolic-ref HEAD >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done