Merge branch 'ef/checkout-empty' into maint

Running "git checkout" on an unborn branch used to corrupt HEAD
(regression in 1.7.10); this makes it error out.

By Erik Faye-Lund
* ef/checkout-empty:
  checkout: do not corrupt HEAD on empty repo

Author: gitster

builtin/checkout.c

@@ -1091,7 +1091,7 @@ 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