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>

builtin/checkout.c

@@ -874,7 +874,7 @@ no_reference:
 		if (strbuf_check_branch_ref(&buf, opts.new_branch))
 			die("git checkout: we do not like '%s' as a branch name.",
 			    opts.new_branch);
-		if (!get_sha1(buf.buf, rev)) {
+		if (ref_exists(buf.buf)) {
 			opts.branch_exists = 1;
 			if (!opts.new_branch_force)
 				die("git checkout: branch %s already exists",