Merge branch 'jc/merge-bases'

Optimise the "merge-base" computation a bit, and also update its users that do not need the full merge-base information to call a cheaper subset. * jc/merge-bases: reduce_heads(): reimplement on top of remove_redundant() merge-base: "--is-ancestor A B" get_merge_bases_many(): walk from many tips in parallel in_merge_bases(): use paint_down_to_common() merge_bases_many(): split out the logic to paint history in_merge_bases(): omit unnecessary redundant common ancestor reduction http-push: use in_merge_bases() for fast-forward check receive-pack: use in_merge_bases() for fast-forward check in_merge_bases(): support only one "other" commit