I'm surprised to find that git cat-file --batch, on a Linux system,
strips the \r from an input like "HEAD:foo\r\n"

It's obvious, of course, that it will remove the newline, and so this
interface cannot be used to query about a filename that, for some
horrible reason[1], contains a newline. But very surprising that it
cannot be used for filename that contains a carriage return, at least
on a non-Windows system.

The docs for cat-file --batch say the list of objects is separated by
linefeeds. I don't know if updating the docs is the best fix.
(I'd be happy to use a -z if it had one.)

-- 
see shy jo

[1] aka "a large enough number of monkeys"