builtin/repack.c: extract `has_existing_non_kept_packs()`

When there is: - at least one pre-existing packfile (which is not marked as kept), - repacking with the `-d` flag, and - not doing a cruft repack , then we pass a handful of additional options to the inner `pack-objects` process, like `--unpack-unreachable`, `--keep-unreachable`, and `--pack-loose-unreachable`, in addition to marking any packs we just wrote for promisor remotes as kept in-core (with `--keep-pack`, as opposed to the presence of a ".keep" file on disk). Because we store both cruft and non-cruft packs together in the same `existing.non_kept_packs` list, it suffices to check its `nr` member to see if it is zero or not. But a following change will store cruft- and non-cruft packs separately, meaning this check would break as a result. Prepare for this by extracting this part of the check into a new helper function called `has_existing_non_kept_packs()`. This patch does not introduce any functional changes, but prepares us to make a more isolated change in a subsequent patch. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>