Merge branch 'zk/push-use-bitmaps'

"git push" sometimes perform poorly when reachability bitmaps are used, even in a repository where other operations are helped by bitmaps. The push.useBitmaps configuration variable is introduced to allow disabling use of reachability bitmaps only for "git push". * zk/push-use-bitmaps: send-pack.c: add config push.useBitmaps