test-gendoc: mute gendoc stderr
Solaris had problems with zh_CN and warned "couldn't set locale correctly" on
stderr. We don't care.
# should print nothing
# should fail
abort: path 'b/x' is inside repo 'b'
abort: path 'b/x' is inside repo 'b'
# should fail
abort: path 'b/x' is inside repo 'b'
# should arguably print nothing
# should fail
abort: path 'b/a' is inside repo 'b'