check-code: add a rule against a GNU sed-ism BSD sed requires the 'i' command to be followed with a backslash and a newline, like so: $ sed -e '/^@/i\ > other' We've encountered this problem before, e.g. in test-mq.t (900767dfa80d). This change adds a check-code rule and fixes two instances of the problem in test-record.t.