protocol-caps: check object existence regardless of the attributes requested

protocol-caps: check object existence regardless of the attributes requested

Currently, send_info() only checks for existence when the attribute
'size' is also requested. Requesting a bare OID, without attributes only
echoes back the OID.

Extract the existence check to be done regardless of the number of
attributes requested.

While at it, introduce a wrapper called get_object_info() similar to
odb_read_object_info() that returns OBJ_BAD on fail and adds
OBJECT_INFO_SKIP_FETCH_OBJECT and OBJECT_INFO_QUICK flags.
OBJECT_INFO_SKIP_FETCH_OBJECT is so a server with a partial clone
doesn't trigger fetching objects when it gets an object-info request
with an OID that is not available locally. A server should only report
what it has locally.

Tighten the condition used to determine whether an object is
recognized. get_object_info() returns OBJ_BAD for unknown objects,
but OBJ_NONE (0) can also mean "not found". Change the check from '< 0'
to '<= OBJ_NONE' to cover both as unrecognized.

With this patch, a bare OID has two possible responses:

1. Recognized OID: the server answers with "<OID>"

2. Unrecognized OID: the server answers with "<OID> SP"

Update the object-info section in 'gitprotocol-v2.adoc':
- Require full obj-oid explicitly.
- Fix parentheses.
- Define obj-size explicitly.
- Make obj-size optional in obj-info and document the behavior
  for unrecognized object IDs.
- Describe the attr header as zero or more pkt-lines, one per attribute,
  matching what the server implements. A request with no attributes gets
  no header.

Signed-off-by: Pablo Sabater <pabloosabaterr@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>