sha1_file.c: make pack-name helper globally accessible

sha1_file.c: make pack-name helper globally accessible

We provide sha1_pack_name() and sha1_pack_index_name(), but
the more generic form (which takes its own strbuf and an
arbitrary extension) is only used to implement the other
two.  Let's make it available, but clean up a few things:

  1. Name it odb_pack_name(), as the original
     sha1_get_pack_name() is long but not all that
     descriptive.

  2. Switch the strbuf argument to the beginning, so that it
     matches similar path-building functions like
     git_path_buf().

  3. Clean up the out-dated docstring and move it to the
     public declaration.

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