Merge branch 'jc/maint-1.7.3-checkout-describe' into maint

* jc/maint-1.7.3-checkout-describe:
  checkout -b <name>: correctly detect existing branch

builtin/checkout.c

@@ -1071,7 +1071,7 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 		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"),

refs.c

@@ -1826,6 +1826,12 @@ int update_ref(const char *action, const char *refname,
 	return 0;
 }
 
+int ref_exists(char *refname)
+{
+	unsigned char sha1[20];
+	return !!resolve_ref(refname, sha1, 1, NULL);
+}
+
 struct ref *find_ref_by_name(const struct ref *list, const char *name)
 {
 	for ( ; list; list = list->next)

refs.h

@@ -54,6 +54,7 @@ extern void warn_dangling_symref(FILE *fp, const char *msg_fmt, const char *refn
  */
 extern void add_extra_ref(const char *refname, const unsigned char *sha1, int flags);
 extern void clear_extra_refs(void);
+extern int ref_exists(char *);
 
 extern int peel_ref(const char *, unsigned char *);
 

t/t2018-checkout-branch.sh

@@ -169,4 +169,15 @@ 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