builtin/repack.c: do not repack single packs with --geometric

builtin/repack.c: do not repack single packs with --geometric

In 0fabafd0b9 (builtin/repack.c: add '--geometric' option, 2021-02-22),
the 'git repack --geometric' code aborts early when there is zero or one
pack.

When there are no packs, this code does the right thing by placing the
split at "0". But when there is exactly one pack, the split is placed at
"1", which means that "git repack --geometric" (with any factor)
repacks all of the objects in a single pack.

This is wasteful, and the remaining code in split_pack_geometry() does
the right thing (not repacking the objects in a single pack) even when
only one pack is present.

Loosen the guard to only stop when there aren't any packs, and let the
rest of the code do the right thing. Add a test to ensure that this is
the case.

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