After a partial clone, repeated fetches from promisor remote would
have accumulated many packfiles marked with .promisor bit without
getting them coalesced into fewer packfiles, hurting performance.
"git repack" now learned to repack them.
* jt/repack-promisor-packs:
repack: repack promisor objects if -a or -A is set
repack: refactor setup of pack-objects cmd