odb_pack_keep(): stop generating keepfile name

odb_pack_keep(): stop generating keepfile name

The odb_pack_keep() function generates the name of a .keep
file and opens it. This has two problems:

  1. It requires a fixed-size buffer to create the filename
     and doesn't notice when the result is truncated.

  2. Of the two callers, one sometimes wants to open a
     filename it already has, which makes things awkward (it
     has to do so manually, and skips the leading-directory
     creation).

Instead, let's have odb_pack_keep() just open the file.
Generating the name isn't hard, and a future patch will
switch callers over to odb_pack_name() anyway.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>