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.

contrib/check-code.py

@@ -74,6 +74,8 b' testpats = ['
     (r'/dev/u?random', "don't use entropy, use /dev/zero"),
     (r'do\s*true;\s*done', "don't use true as loop body, use sleep 0"),
     (r'^( *)\t', "don't use tabs to indent"),
+    (r'sed .*\'/[^/]*/i[^\\][^\n]',
+     "put a backslash-escaped newline after sed 'i' command"),
   ],
   # warnings
   [

tests/test-record.t

@@ -1039,7 +1039,8 b' Abort early when a merge is in progress'
 Editing patch (and ignoring trailing text)
 
   $ cat > editor.sh << '__EOF__'
-  > sed -e 7d -e '5s/^-/ /' -e '/^# ---/itrailing\nditto' "$1" > tmp
+  > sed -e 7d -e '5s/^-/ /' -e '/^# ---/i\
+  > trailing\nditto' "$1" > tmp
   > mv tmp "$1"
   > __EOF__
   $ cat > editedfile << '__EOF__'
@@ -1204,7 +1205,8 b' Malformed patch - error handling'
 random text in random positions is still an error
 
   $ cat > editor.sh << '__EOF__'
-  > sed -e '/^@/iother' "$1" > tmp
+  > sed -e '/^@/i\
+  > other' "$1" > tmp
   > mv tmp "$1"
   > __EOF__
   $ HGEDITOR="\"sh\" \"`pwd`/editor.sh\"" hg record <<EOF