Multiline search strings are weirdly broken (inserting control characters
in the command line) and probably not very useful anyway.
On the other hand I often want to compose a multi-line command
from single-line commands I ran previously.
Let's support this case by limiting the initial search string to the current
line; and replace only that line.
Alternatively this could operate on jobs (that is, replace a surrounding
"foo | bar") instead of using line boundaries.
