It turns out that if there are text anchors (that is, \A or \z, or ^/$
when multi-line is disabled), then the "fast" line searching path isn't
quite correct. Since searching without multi-line mode is exceptionally
rare, we just look for the presence of text anchors and specifically
disable the line terminator option in 'grep-regex'. This in turn
inhibits the "fast" line searching path.
Fixes#2260
8e57989cd2