check-code: fix sed 'i' command rule newline matching

The regular expression was meant to match cases where an 'i' command was
not followed by precisely a '\' and then a newline; it failed to match
the newline, so cases with a '\' but no newline would erroneously pass.

--HG--
branch : stable
tests: make sed usage in test-unionrepo.t cross-platform

Usage of the 'i' command proves tricky. I tried to write a check-code rule, but
failed.

--HG--
branch : stable
extra : rebase_source : cb3e4e9df7cb5fff90f6b43ab262ab1b7679d72e