Updated git-gc --auto series.

An updated series of "git-gc --auto", on top of 'next', consisting of 8 patches, will follow this message.

Differences from the previous round are:

- Earlier if you have too many unreachable loose objects, automated gc would have tried to "repack -d -l" which would not improve the situation at all every time it was run. It now at least warns upon such a situation;

- pack-objects learned --keep-unreachable option which helps "repack -a -d" not to lose packed but unreachable objects while repacking existing packs into a new pack;

- repack learned -A option which is similar to -a but gives --keep-unreachable to underlying pack-objects;

- "git-gc --auto" runs "git-repack -A -d -l" when there are too many packs in the repository;

- These changes are documented ;-)