ab/only-single-progress-at-once (was: What's cooking in git.git (Dec 2021, #06; Mon, 27))

> * ab/only-single-progress-at-once (2021-11-03) 8 commits
>  - progress.c: add & assert a "global_progress" variable
>  - various *.c: use isatty(0|2), not isatty(STDIN_FILENO|STDERR_FILENO)
>  - pack-bitmap-write.c: don't return without stop_progress()
>  - progress.c: add temporary variable from progress struct
>  - progress.c tests: test some invalid usage
>  - progress.c tests: make start/stop commands on stdin
>  - progress.c test helper: add missing braces
>  - leak tests: fix a memory leaks in "test-progress" helper
>
>  Further tweaks on progress API.
>
>  Needs review.
>  The last three rounds has seen little reaction, even though earlier
>  round saw a lot of responses. The latest round needs a serious
>  review or at least Acks from past commentors.
>  source: <cover-v6-0.8-00000000000-20211102T122507Z-avarab@gmail.com>