checkout -b <name>: correctly detect existing branch

When create a new branch, we fed "refs/heads/<proposed name>" as a string
to get_sha1() and expected it to fail when a branch already exists.

The right way to check if a ref exists is to check with resolve_ref().

A naïve solution that might appear attractive but does not work is to
forbid slashes in get_describe_name() but that will not work. A describe
name is is in the form of "ANYTHING-g<short sha1>", and that ANYTHING part
comes from a original tag name used in the repository the user ran the
describe command. A sick user could have a confusing hierarchical tag
whose name is "refs/heads/foobar" (stored as refs/tags/refs/heads/foobar")
to generate a describe name "refs/heads/foobar-6-g02ac983", and we should
be able to use that name to refer to the object whose name is 02ac983.

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

1 files changed, 11 insertions, 0 deletions

diff --git a/t/t2018-checkout-branch.sh b/t/t2018-checkout-branch.sh
index 1caffeac0..741d84257 100755
--- a/t/t2018-checkout-branch.sh
+++ b/t/t2018-checkout-branch.sh
@@ -163,4 +163,15 @@ test_expect_success 'checkout -f -B to an existing branch with mergeable changes
 	test_must_fail test_dirty_mergeable
 '
 
+test_expect_success 'checkout -b <describe>' '
+	git tag -f -m "First commit" initial initial &&
+	git checkout -f change1 &&
+	name=$(git describe) &&
+	git checkout -b $name &&
+	git diff --exit-code change1 &&
+	echo "refs/heads/$name" >expect &&
+	git symbolic-ref HEAD >actual &&
+	test_cmp expect actual
+'
+
 test_done