Re: git 2.36.0 regression: pre-commit hooks no longer have stdout/stderr as tty

> 
> here's the shortest reproduction --
> 
> ```console
> $ cat ../testrepo/.git/hooks/pre-commit
> #!/usr/bin/env bash
> if [ -t 1 ]; then
>     echo GOOD
> fi
> ```
> 
> in previous git versions:
> 
> ```
> $ git commit -q --allow-empty -m foo
> GOOD
> $
> ```
> 
> with git 2.36.0:
> 
> ````
> $ git commit -q --allow-empty -m foo
> $
> ```
> 
> why I care: I maintain a git hooks framework which uses `isatty` to
> detect whether it's appropriate to color the output.  many tools
> utilize the same check.  in 2.36.0+ isatty is false for stdout and
> stderr causing coloring to be turned off.
> 
> I bisected this (it was a little complicated, needed to force a pty):
> 
> `../testrepo`: a git repo set up with the hook above
> 
> `../bisect.sh`:
> 
> ```bash
> #!/usr/bin/env bash
> set -eux
> git clean -fxfd >& /dev/null
> make -j6 prefix="$PWD/prefix" NO_GETTEXT=1 NO_TCLTK=1 install >& /dev/null
> export PATH="$PWD/prefix/bin:$PATH"
> cd ../testrepo
> (../pty git commit -q --allow-empty -m foo || true) | grep GOOD
> ```
> 
> `../pty`:
> 
> ```python
> #!/usr/bin/env python3
> import errno
> import os
> import subprocess
> import sys
> 
> x: int = 'nope'
> 
> 
> class Pty(object):
>     def __init__(self):
>         self.r = self.w = None
> 
>     def __enter__(self):
>         self.r, self.w = os.openpty()
> 
>         return self
> 
>     def close_w(self):
>         if self.w is not None:
>             os.close(self.w)
>             self.w = None
> 
>     def close_r(self):
>         assert self.r is not None
>         os.close(self.r)
>         self.r = None
> 
>     def __exit__(self, exc_type, exc_value, traceback):
>         self.close_w()
>         self.close_r()
> 
> 
> def cmd_output_p(*cmd, **kwargs):
>     with open(os.devnull) as devnull, Pty() as pty:
>         kwargs = {'stdin': devnull, 'stdout': pty.w, 'stderr': pty.w}
>         proc = subprocess.Popen(cmd, **kwargs)
>         pty.close_w()
> 
>         buf = b''
>         while True:
>             try:
>                 bts = os.read(pty.r, 4096)
>             except OSError as e:
>                 if e.errno == errno.EIO:
>                     bts = b''
>                 else:
>                     raise
>             else:
>                 buf += bts
>             if not bts:
>                 break
> 
>     return proc.wait(), buf, None
> 
> 
> if __name__ == '__main__':
>     _, buf, _ = cmd_output_p(*sys.argv[1:])
>     sys.stdout.buffer.write(buf)
> ```
> 
> the first commit it points out:
> 
> ```
> f443246b9f29b815f0b98a07bb2d425628ae6522 is the first bad commit
> commit f443246b9f29b815f0b98a07bb2d425628ae6522
> Author: Emily Shaffer <emilyshaffer@google.com>
> 
>     commit: convert {pre-commit,prepare-commit-msg} hook to hook.h
> 
>     Move these hooks hook away from run-command.h to and over to the new
>     hook.h library.
> 
>     Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
>     Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
>     Acked-by: Emily Shaffer <emilyshaffer@google.com>
>     Signed-off-by: Junio C Hamano <gitster@pobox.com>
> 
>  commit.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
> bisect run success
> ```