archive: re-allow HEAD:Documentation on a remote invocation

archive: re-allow HEAD:Documentation on a remote invocation

The tightening done in (ee27ca4a: archive: don't let remote clients
get unreachable commits, 2011-11-17) went too far and disallowed
HEAD:Documentation as it would try to find "HEAD:Documentation" as a
ref.

Only DWIM the "HEAD" part to see if it exists as a ref. Once we're
sure that we've been given a valid ref, we follow the normal code
path. This still disallows attempts to access commits which are not
branch tips.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>