t/t5571-prep-push-hook.sh: Add test with writing to stderr

[t-b]

Git v2.53.0-rc0 included f406b89 (Merge branch 'ar/run-command-hook', 2026-01-06), which caused a regression on Windows. While this merge was reverted for independent reasons in a3d1f39 (Revert "Merge branch 'ar/run-command-hook'", 2026-01-15), it seems worthwhile to ensure that writing to standard error from a pre-push hook remains unbroken.

The symptom, when running this regression test case against v2.53.0-rc0.windows.1 is that the git push fails, with this message printed to standard error:

.git/hooks/pre-push: line 2: /dev/stderr: No such file or direct[61/1940]
error: failed to push some refs to 'repo1'

When that hook runs, /dev/stderr is a symlink to /proc/self/fd/2, as always, but ls -l /proc/self/fd/ shows this in the failing run

total 0
lrwxrwxrwx 1 me 4096 0 Jan 27 14:34 0 -> pipe:[0]
lrwxrwxrwx 1 me 4096 0 Jan 27 14:34 1 -> pipe:[0]
lrwxrwxrwx 1 me 4096 0 Jan 27 14:34 2 -> pipe:[0]

instead of the expected contents (which are shown when running this against v2.53.0-rc1.windows.1):

total 0
lrwxrwxrwx 1 me 4096 0 Jan 27 14:53 0 -> 'pipe:[0]'
lrwxrwxrwx 1 me 4096 0 Jan 27 14:53 1 -> /dev/cons1
lrwxrwxrwx 1 me 4096 0 Jan 27 14:53 2 -> '/path/to/git/t/trash directory.t5571-pre-push-hook/actual'

This suggests that the underlying reason might be that stdout has an exclusive handle to that pipe, and opening stderr (which points to the same pipe) fails because of that exclusively-opened stdout handle.

This closes #6053.

[dscho]

I investigated a bit more whether I could pinpoint an exact upstream commit that would be responsible for this, but could not quite figure it out. I left the conclusions of my analysis in the description of this PR, which will become the merge commit message.