cat-file: handle content request for --batch-command without type

The batch mode of cat-file needs to know the object's type in order to
print the contents (because it decides whether to stream or not based on
object type). The default batch output contains %(objecttype), so we get
the type info automatically. But when it doesn't, we have to ask for it
explicitly.

In the --batch code path, we check while setting up the object_info
struct whether we will print the contents, and if so set "typep" to get
the value. This comes from 6554dfa97a (cat-file: handle --batch format
with missing type/size, 2013-12-12).

But later we added a --batch-command mode, which does not do the same
trick. The decision about whether to retrieve the contents is made
per-command (a "contents" vs "info" command), so we can't decide when
building the object_info originally. As a result, asking for:

  echo "contents HEAD" | git cat-file --batch-command="%(objectname)"

will fail the assertion in print_object_or_die() that the type was
actually filled in.

We can fix it by tweaking the object_info on the fly as we receive each
command. But we should be careful to restore it afterwards; otherwise a
sequence of commands like:

  contents $one
  info $two
  info $three

will pay the type-lookup price for $two and $three when it does not need
to. This wouldn't be incorrect, but just slightly inefficient (and hence
there are no tests for that part, because the externally-visible
behavior is the same).

Reported-by: Alan Stokes <alan@source.dev>
Helped-by: Pablo Sabater <pabloosabaterr@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>