clone: narrow success tests on HEAD-less remotes

Only allow the remote default branch checking to fail when the remote
default branch doesn't exist (`git_remote__default_branch` returns
`GIT_ENOTFOUND`). If there was any other type of error - for example, an
allocation failure - we should not swallow that and continue to fail.

This allows us to support the case when a remote has not advertised a
HEAD -- this is possible, for example, when the remote has constrained
the caller to a particular namespace. But other remote failures remain
as failures.

src/libgit2/clone.c

@@ -282,7 +282,11 @@ static int update_head_to_branch(
 			reflog_message)) < 0)
 		goto cleanup;
 
-	if (git_remote__default_branch(&default_branch, remote) < 0)
+	retcode = git_remote__default_branch(&default_branch, remote);
+
+	if (retcode == GIT_ENOTFOUND)
+		retcode = 0;
+	else if (retcode)
 		goto cleanup;
 
 	if (!git_remote__matching_refspec(remote, git_str_cstr(&default_branch)))