builtin/repack.c: provide pack locations to `generated_pack_install()`

builtin/repack.c: provide pack locations to `generated_pack_install()`

Repeat what was done in the preceding commit for the
`generated_pack_install()` function, which needs both "packdir" and
"packtmp".

(As an aside, it is somewhat unfortunate that the final three parameters
to this function are all "const char *", making errors like passing
"packdir" and "packtmp" in the wrong order easy. We could define a new
structure here, but that may be too heavy-handed.)

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>