Merge branch 'jc/branch-name-sanity'

"git branch" and "git checkout -b" are now forbidden from creating
a branch whose name is "HEAD".

* jc/branch-name-sanity:
  builtin/branch: remove redundant check for HEAD
  branch: correctly reject refs/heads/{-dash,HEAD}
  branch: split validate_new_branchname() into two
  branch: streamline "attr_only" handling in validate_new_branchname()

sha1_name.c

@@ -1438,9 +1438,19 @@ int strbuf_check_branch_ref(struct strbuf *sb, const char *name)
 		strbuf_branchname(sb, name, INTERPRET_BRANCH_LOCAL);
 	else
 		strbuf_addstr(sb, name);
-	if (name[0] == '-')
-		return -1;
+
+	/*
+	 * This splice must be done even if we end up rejecting the
+	 * name; builtin/branch.c::copy_or_rename_branch() still wants
+	 * to see what the name expanded to so that "branch -m" can be
+	 * used as a tool to correct earlier mistakes.
+	 */
 	strbuf_splice(sb, 0, 0, "refs/heads/", 11);
+
+	if (*name == '-' ||
+	    !strcmp(sb->buf, "refs/heads/HEAD"))
+		return -1;
+
 	return check_refname_format(sb->buf, 0);
 }