git: use --batch-command in git 2.36+ to save processes
`git cat-file --batch-command' combines the functionality of
`--batch' and `--batch-check' into a single process. This
reduces the amount of running processes and is primarily
useful for coderepos (e.g. solver).
This also fixes prior use of `print { $git->{out} }' which is
a a potential (but unlikely) bug since commit
d4ba8828ab23f278
(git: fix asynchronous batching for deep pipelines, 2023-01-04)
Lack of libgit2 on one of my test machines also uncovered fixes
necessary for t/imapd.t, t/nntpd.t and t/nntpd-v2.t.