Allow builtin-fetch to work on a detached HEAD

Allow builtin-fetch to work on a detached HEAD

If we are running fetch in a repository that has a detached HEAD
then there is no current_branch available.  In such a case any ref
that the fetch might update by definition cannot also be the current
branch so we should always bypass the "don't update HEAD" test.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>