repack: move write_cruft_pack() out of the builtin

In an identical fashion as the previous commit, move the function
`write_cruft_pack()` into its own compilation unit, and make the
function visible through the repack.h API.

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

meson.build

@@ -463,6 +463,7 @@ libgit_sources = [
   'reftable/writer.c',
   'remote.c',
   'repack.c',
+  'repack-cruft.c',
   'repack-filtered.c',
   'repack-geometry.c',
   'repack-midx.c',