sha1_file: silence sha1_loose_object_info

sha1_object_info() returns -1 (OBJ_BAD) if it cannot find the object
for some reason, which suggests that it wants the _caller_ to report
this error.  However, part of its work happens in
sha1_loose_object_info, which _does_ report errors itself.  This is
doubly strange because:

* packed_object_info(), which is the other half of the duo, does _not_
  report this.

* In the event that an object is packed and pruned while
  sha1_object_info_extended() goes looking for it, we would
  erroneously show the error -- even though the code of the latter
  function purports to handle this case gracefully.

* A caller might invoke sha1_object_info() to find the type of an
  object even if that object is not known to exist.

Silence this error.  The others remain untouched as a corrupt object
is a much more grave error than it merely being absent.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -2140,7 +2140,7 @@ static int sha1_loose_object_info(const unsigned char *sha1, unsigned long *size
 
 	map = map_sha1_file(sha1, &mapsize);
 	if (!map)
-		return error("unable to find %s", sha1_to_hex(sha1));
+		return -1;
 	if (unpack_sha1_header(&stream, map, mapsize, hdr, sizeof(hdr)) < 0)
 		status = error("unable to unpack %s header",
 			       sha1_to_hex(sha1));