git-repack --max-pack-size

This patchset adds git-fast-import's "--max-pack-size=N" option, with the same behavior, to git-repack.

This latest patchset includes the following new updates:
* sha1close() changes split out as an earlier patch;
* rebased on top of recent changes to pack-objects;
* fixed delta reuse _reporting_ buglet when deltas split;
* temporary packfiles built in GIT_OBJECT_DIRECTORY not GIT_DIR;
* updated documentation.