repack: do not fall back to incremental repacking with [-a|-A]

repack: do not fall back to incremental repacking with [-a|-A]

When repack is called with either the -a or -A option, the user has
requested to repack all objects including those referenced by the
alternates mechanism. Currently, if there are no local packs without
.keep files, then repack will call pack-objects with the
'--unpacked --incremental' options which causes it to exclude alternate
packed objects. So, remove this fallback.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>