clone: fix up delay cloning conditions

clone: fix up delay cloning conditions

6f48d39 (clone: delay cloning until after remote HEAD checking -
2012-01-16) allows us to perform some checks on remote refs before the
actual cloning happens. But not all transport types support
this. Remote helper with "import" capability will not return complete
ref information until fetch is performed and therefore the clone cannot
be delayed.

foreign_vcs field in struct remote was used to detect this kind of transport
and save the result. This is a mistake because foreign_vcs is designed
to override url-based transport detection. As a result, if the same
"struct transport *" object is used on many different urls and one of
them attached remote transport, the following urls will be mistakenly
attached to the same transport. This fault is worked around by dad0b3d
(push: do not let configured foreign-vcs permanently clobbered -
2012-01-23)

To fix this, detect incomplete refs from transport_get_remote_refs()
by SHA-1. Incomplete ones must have null SHA-1 (*). Then revert
changes related to foreign_cvs field in 6f48d39 and dad0b3d.

A good thing from this change is that cloning smart http transport can
also be delayed. Earlier it falls into the same category "remote
transport, no delay".

(*) Theoretically if one of the remote refs happens to have null SHA-1,
it will trigger false alarm and the clone will not be delayed. But
that chance may be too small for us to pay attention to.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>