pack-objects: won't show the "Writing objects" progress if bitmap exists

From: Xia XiaoWen <redacted>

If bitmap exists, Run the command:

    $ git pack-objects --all-progress-implied --all --stdout --thin
--delta-base-offset --progress

Won't show `Writing objects` progress until writing objects finished.
The output will display `Enumerating objects` for an long time(about
100s in my case):

    Enumerating objects: 329780, done.

Untils writing objects finished, then the output display like this:

    Writing objects: 100% (329780/329780), 110.26 MiB | 1.02 MiB/s, done.
    Total 329780 (delta 0), reused 0 (delta 0), pack-reused 329780

I used `git repack -ad` to create packfile bitmap in the bare repo
(git.git in my case).

I found the "problem"(maybe) while `git push --mirror` to github.