gc.sh: avoid redundant repacks in combine_small_packs

gc.sh: avoid redundant repacks in combine_small_packs

When a fork parent calls "combine_small_packs 1" before copying packs
down to its forks, the idea is to force even a single small pack to get
new deltas so the forks do not get suboptimal deltas.

However, if girocco.redelta is set to false this is pointless since no
new deltas will be generated for a single pack.

Also the logic was previously such that two (or more) small packs would
be combined into one and then that one resulting pack would
unnecessarily have its deltas regenerated again when the flag was set.

Tweak the logic when the redelta-a-single-pack flag is set to avoid a
superfluous redelta in the more-than-one-small-pack-is-present case.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>