Non-zero exit code of custom filter process is not handled

Hello everyone,

I would like to report https://github.com/gitgitgadget/git/issues/1516 .

When a custom filter process exits with a non-zero code, the git currently tries to decode the response, even if it should fail and let the user know the problem is not the payload, but instead of the filter process.

You can reproduce this with setting a git filter to "php non-existent.php", in this case the filter (php binary) will exit with non-zero code. Currently "fatal: protocol error: bad line length character: Coul" is printed which is very hard to understand as even not the whole error string is shown.

Thank you for looking into this.

With kind regards / Mit freundlichen Grüßen / S přátelským pozdravem,

Michael Voříšek, student ČVUT FJFI