hgweb: make help verbose again (issue3899)

Due to regression introduced in f5db3092790f, help in hgweb
was rendered in non-verbose form (issue3899)

--HG--
branch : stable
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.

--HG--
branch : stable