builtin/repack.c: rename variables that deal with non-kept packs

builtin/repack.c: rename variables that deal with non-kept packs

The new variable `existing_kept_packs` (and corresponding parameter
`fname_kept_list`) added by the previous patch make it seem like
`existing_packs` and `fname_list` are each subsets of the other two
respectively.

In reality, each pair is disjoint: one stores the packs without .keep
files, and the other stores the packs with .keep files. Rename each to
more clearly reflect this.

Suggested-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>