git-repack --max-pack-size

The three most common ways of making large packfiles are git-fast-import, the first git-repack, or git-repack -a. The first already supports a "--max-pack-size=N" option, which limits the resulting packfiles to N megabytes. This patchset adds the same option, with the same behavior, to git-repack to handle the other two cases.

This iteration fixes miscellaneous issues discussed on the list and introduces no behavior not seen elsewhere in git.
It is based on "next" in order to incorporate Nicolas Pitre's work.