submodule--helper: teach push-check to handle HEAD

submodule--helper: teach push-check to handle HEAD

In 06bf4ad1d (push: propagate remote and refspec with
--recurse-submodules) push was taught how to propagate a refspec down to
submodules when the '--recurse-submodules' flag is given.  The only refspecs
that are allowed to be propagated are ones which name a ref which exists
in both the superproject and the submodule, with the caveat that 'HEAD'
was disallowed.

This patch teaches push-check (the submodule helper which determines if
a refspec can be propagated to a submodule) to permit propagating 'HEAD'
if and only if the superproject and the submodule both have the same
named branch checked out and the submodule is not in a detached head
state.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>