pack-objects: allow setting `pack.allowPackReuse` to "single"

pack-objects: allow setting `pack.allowPackReuse` to "single"

In e704fc7978 (pack-objects: introduce pack.allowPackReuse, 2019-12-18),
the `pack.allowPackReuse` configuration option was introduced, allowing
users to disable the pack reuse mechanism.

To prepare for debugging multi-pack reuse, allow setting configuration
to "single" in addition to the usual bool-or-int values.

"single" implies the same behavior as "true", "1", "yes", and so on. But
it will complement a new "multi" value (to be introduced in a future
commit). When set to "single", we will only perform pack reuse on a
single pack, regardless of whether or not there are multiple MIDX'd
packs.

This requires no code changes (yet), since we only support single pack
reuse.

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