builtin/repack.c: clean up unused `#include`s

builtin/repack.c: clean up unused `#include`s

Over the past several dozen commits, we have moved a large amount of
functionality out of the repack builtin and into other files like
repack.c, repack-cruft.c, repack-filtered.c, repack-midx.c, and
repack-promisor.c.

These files specify the minimal set of `#include`s that they need to
compile successfully, but we did not change the set of `#include`s in
the repack builtin itself.

Now that the code movement is complete, let's clean up that set of
`#include`s and trim down the builtin to include the minimal amount of
external headers necessary to compile.

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