repack: remove pack_geometry API from the builtin

Now that the pack_geometry API is fully factored and isolated from the
rest of the builtin, declare it within repack.h and move its
implementation to "repack-geometry.c" as a separate component.

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-geometry.c',
   'repack-promisor.c',
   'replace-object.c',
   'repo-settings.c',